In my class, I was playing around and found out that CSS works with made-up elements.
Example:
imsocool {
color:blue;
}
<imsocool>HELLO</imsocool>
When my professor first saw me using this, he was a bit surprised that made-up elements worked and recommended I simply change all of my made up elements to paragraphs with ID's.
Why doesn't my professor want me to use made-up elements? They work effectively.
Also, why didn't he know that made-up elements exist and work with CSS. Are they uncommon?
Why does CSS work with fake elements?
(Most) browsers are designed to be (to some degree) forward compatible with future additions to HTML. Unrecognised elements are parsed into the DOM, but have no semantics or specialised default rendering associated with them.
When a new element is added to the specification, sometimes CSS, JavaScript and ARIA can be used to provide the same functionality in older browsers (and the elements have to appear in the DOM for those languages to be able to manipulate them to add that functionality).
(There is a specification for custom elements, but they have specific naming requirements and require registering using JavaScript.)
Why doesn't my professor want me to use made-up elements?
They are not allowed by the HTML specification
They might conflict with future standard elements with the same name
There is probably an existing HTML element that is better suited to the task
Also; why didn't he know that made-up elements existed and worked with CSS. Are they uncommon?
Yes. People don't use them because they have the above problems.
TL;DR
Custom tags are invalid in HTML. This may lead to rendering issues.
Makes future development more difficult since code is not portable.
Valid HTML offers a lot of benefits such as SEO, speed, and professionalism.
Long Answer
There are some arguments that code with custom tags is more usable.
However, it leads to invalid HTML. Which is not good for your site.
The Point of Valid CSS/HTML | StackOverflow
Google prefers it so it is good for SEO.
It makes your web page more likely to work in browsers you haven't tested.
It makes you look more professional (to some developers at least)
Compliant browsers can render [valid HTML faster]
It points out a bunch of obscure bugs you've probably missed that affect things you probably haven't tested e.g. the codepage or language set of the page.
Why Validate | W3C
Validation as a debugging tool
Validation as a future-proof quality check
Validation eases maintenance
Validation helps teach good practices
Validation is a sign of professionalism
YADA (yet another (different) answer)
Edit: Please see the comment from BoltClock below regarding type vs tag vs element. I usually don't worry about semantics but his comment is very appropriate and informative.
Although there are already a bunch of good replies, you indicated that your professor prompted you to post this question so it appears you are (formally) in school. I thought I would expound a little bit more in depth about not only CSS but also the mechanics of web browsers. According to Wikipedia, "CSS is a style sheet language used for describing ... a document written in a markup language." (I added the emphasis on "a") Notice that it doesn't say "written in HTML" much less a specific version of HTML. CSS can be used on HTML, XHTML, XML, SGML, XAML, etc. Of course, you need something that will render each of these document types that will also apply styling. By definition, CSS does not know / understand / care about specific markup language tags. So, the tags may be "invalid" as far as HTML is concerned, but there is no concept of a "valid" tag/element/type in CSS.
Modern visual browsers are not monolithic programs. They are an amalgam of different "engines" that have specific jobs to do. At a bare minimum I can think of 3 engines, the rendering engine, the CSS engine, and the javascript engine/VM. Not sure if the parser is part of the rendering engine (or vice versa) or if it is a separate engine, but you get the idea.
Whether or not a visual browser (others have already addressed the fact that screen readers might have other challenges dealing with invalid tags) applies the formatting depends on whether the parser leaves the "invalid" tag in the document and then whether the rendering engine applies styles to that tag. Since it would make it more difficult to develop/maintain, CSS engines are not written to understand that "This is an HTML document so here are the list of valid tags / elements / types." CSS engines simply find tags / elements / types and then tell the rendering engine, "Here are the styles you should apply." Whether or not the rendering engine decides to actually apply the styles is up it.
Here is an easy way to think of the basic flow from engine to engine: parser -> CSS -> rendering. In reality it is much more convoluted but this is good enough for starters.
This answer is already too long so I will end there.
Unknown elements are treated as divs by modern browsers. That's why they work. This is part of the oncoming HTML5 standard that introduces a modular structure to which new elements can be added.
In older browsers (I think IE7-) you can apply a Javascript-trick after which they will work as well.
Here is a related question I found when looking for an example.
Here is a question about the Javascript fix. Turns out it is indeed IE7 that doesn't support these elements out of the box.
Also; why didn't he know that made-up tags existed and worked with CSS. Are they uncommon?
Yes, quite. But especially: they don't serve additional purpose. And they are new to html5. In earlier versions of HTML an unknown tag was invalid.
Also, teachers seem to have gaps in their knowledge, sometimes. This might be due to the fact that they need to teach students the basics about a given subject, and it doesn't really pay off to know all ins and outs and be really up to date.
I once got detention because a teacher thought I programmed a virus, just because I could make a computer play music using the play command in GWBasic. (True story, and yes, long ago). But whatever the reason, I think the advice not to use custome elements is a sound one.
Actually you can use custom elements. Here is the W3C spec on this subject:
http://w3c.github.io/webcomponents/spec/custom/
And here is a tutorial explaining how to use them:
http://www.html5rocks.com/en/tutorials/webcomponents/customelements/
As pointed out by #Quentin: this is a draft specification in the early days of development, and that it imposes restrictions on what the element names can be.
There are a few things about the other answers that are either just poorly phrased or perhaps a little incorrect.
FALSE(ish): Non-standard HTML elements are "not allowed", "illegal", or "invalid".
Not necessarily. They're "non-conforming". What's the difference? Something can "not conform" and still be "allowed". The W3C aren't going to send the HTML police to your home and haul you away.
The W3C left things this way for a reason. Conformance and specifications are defined by a community. If you happen to have a smaller community consuming HTML for more specific purposes and they all agree on some new Elements they need to make things easier, they can have what the W3C refers to as "other applicable specifications". (this is a gross over simplification, obviously, but you get the idea)
That said, strict validators will declare your non-standard elements to be "invalid". but that's because the validator's job is to ensure conformance to whatever spec it's validating for, not to ensure "legality" for the browser or for use.
FALSE(ish): Non-standard HTML elements will result in rendering issues
Possibly, but unlikely. (replace "will" with "might") The only way this should result in a rendering issue is if your custom element conflicts with another specification, such as a change to the HTML spec or another specification being honored within the same system (such as SVG, Math, or something custom).
In fact, the reason CSS can style non-standard tags is because the HTML specification clearly states that:
User agents must treat elements and attributes that they do not understand as semantically neutral; leaving them in the DOM (for DOM processors), and styling them according to CSS (for CSS processors), but not inferring any meaning from them
Note: if you want to use a custom tag, just remember a change to the HTML spec at a later time could blow your styling up, so be prepared. It's really unlikely that the W3C will implement the <imsocool> tag, however.
Non-standard tags and JavaScript (via the DOM)
The reason you can access and alter custom elements using JavaScript is because the specification even talks about how they should be handled in the DOM, which is the (really horrible) API that allows you to manipulate the elements on your page.
The HTMLUnknownElement interface must be used for HTML elements that are not defined by this specification (or other applicable specifications).
TL;DR: Conforming to the spec is done for purposes of communication and safety. Non-conformance is still allowed by everything but a validator, whose sole purpose is to enforce conformity, but whose use is optional.
For example:
var wee = document.createElement('wee');
console.log(wee.toString()); //[object HTMLUnknownElement]
(I'm sure this will draw flames, but there's my 2 cents)
According to the specs:
CSS
A type selector is the name of a document language element type written using the syntax of CSS qualified names
I thought this was called the element selector, but apparently it is actually the type selector. The spec goes on to talk about CSS qualified names which put no restriction on what the names actually are. That is to say that as long as the type selector matches CSS qualified name syntax it is technically correct CSS and will match the element in the document. There is no CSS-specific restriction on elements that do not exist in a particular spec -- HTML or otherwise.
HTML
There is no official restriction on including any tags in the document that you want. However, the documentation does say
Authors must not use elements, attributes, or attribute values for purposes other than their appropriate intended semantic purpose, as doing so prevents software from correctly processing the page.
And it later says
Authors must not use elements, attributes, or attribute values that are not permitted by this specification or other applicable specifications, as doing so makes it significantly harder for the language to be extended in the future.
I'm not sure specifically where or if the spec says that unkown elements are allowed, but it does talk about the HTMLUnknownElement interface for unrecognized elements. Some browsers may not even recognize elements that are in the current spec (IE8 comes to mind).
There is a draft for custom elements, though, but I doubt it is implemented anywhere yet.
This is possible with html5 but you need to take into consideration of older browsers.
If you do decide to use them then, make sure to COMMENT your html!! Some people may have some trouble figuring out what it is so a comment could save them a ton of time.
Something like this,
<!-- Custom tags in use, refer to their CSS for aid -->
When you make your own custom tag/elements the older browsers will have no clue what that is just like html5 elements like nav/section.
If you are interested in this concept then I recommend to do it the right way.
Getting started
Custom Elements allow web developers to define new types of HTML
elements. The spec is one of several new API primitives landing under
the Web Components umbrella, but it's quite possibly the most
important. Web Components don't exist without the features unlocked by
custom elements:
Define new HTML/DOM elements Create elements that extend from other
elements Logically bundle together custom functionality into a single
tag Extend the API of existing DOM elements
There is a lot you can do with it and it does make your script beautiful as this article likes to put it. Custom Elements defining new elements in HTML.
So lets recap,
Pros
Very elegant and easy to read.
It is nice to not see so many divs. :p
Allows a unique feel to the code
Cons
Older browser support is a strong thing to consider.
Other developers may have no clue what to do if they don't know about custom tags. (Explain to them or add comments to inform them)
Lastly one thing to take into consideration, but I am unsure, is block and inline elements. By using custom tags you are going to end up writing more css because of the custom tag won't have a default side to it.
The choice is entirely up to you and you should base it on what the project is asking for.
Update 1/2/2014
Here is a very helpful article I found and figured I would share, Custom Elements.
Learn the tech Why Custom Elements? Custom Elements let authors define
their own elements. Authors associate JavaScript code with custom tag
names, and then use those custom tag names as they would any standard
tag.
For example, after registering a special kind of button called
super-button, use the super button just like this:
Custom elements are still elements. We
can create, use, manipulate, and compose them just as easily as any
standard or today.
This seems like a very good library to use but I did notice it didn't pass Window's Build status. This is also in a pre-alpha I believe so I would keep an eye on this while it develops.
Why doesn't he want you to use them? They are not common nor part of the HTML5 standard.
Technically, they are not allowed. They are a hack.
I like them myself, though. You may be interested in XHTML5. It allows you to define your own tags and use them as part of the standard.
Also, as others have pointed out, they are invalid and thus not portable.
Why didn't he know that they exist? I don't know, except that they are not common. Possibly he was just not aware that you could.
Made-up tags are hardly ever used, because it's unlikely that they will work reliably in every current browser, and every future browser.
A browser has to parse the HTML code into elements that it knows, to made-up tags will be converted into something else to fit in the document object model (DOM). As the web standards doesn't cover how to handle everyting that is outside of the standards, web browsers tend to handle non-standars code in different ways.
Web development is tricky enough with a bunch of different browsers that have their own quirks, without adding another element of uncertainty. The best bet it to stick with things that are actually in the standards, that is what the browser vendors try to follow, so that has the best chance to actually work.
I think made-up tags are just potentially more confusing or unclear than p's with IDs (some block of text generally). We all know a p with an ID is a paragraph, but who knows what made-up tags are intended for? At least that's my thought. :) Therefore this is more of a style / clarity issue than one of functionality.
Others have made excellent points but its worth noting that if you look at a framework such as AngularJS, there is a very valid case for custom elements and attributes. These convey not only better semantic meaning to the xml, but they also can provide behavior, look and feel for the web page.
CSS is a style sheet language that can be used to present XML documents, not only (X)HTML documents. Your snippet with the made-up tags could be part of a legal XML document; it would be one if you enclose it in a single root element. Probably you already have a <html> ...</html> around it? Any current browser can display XML documents.
Of course it is not a very good XML document, it lacks a grammar and an XML declaration. If you use an HTML declaration header instead (and probably a server configuration that sends the correct mime type) it would instead be illegal HTML.
(X)HTML has advantages over plain XML as elements have a semantic meaning that is useful in the context of a web page presentation. Tools can work with this semantics, other developers know the meaning, it is less error prone and better to read.
But in other contexts it is better to use CSS with XML and/or XSLT to do the presentation. This is what you did. As this wasn't your task, you didn't know what you were doing, and HTML/CSS is the better way to go most of the time you should stick to it in your scenario.
You should add an (X)HTML header to your document so tools can give you meaningful error messages.
...I simply change all of my made up tags to paragraphs with ID's.
I actually take issue with his suggestion of how to do it properly.
A <p> tag is for paragraphs. I see people using it all the time instead of a div -- simply for spacing purposes or because it seems gentler. If it's not a paragraph, don't use it.
You don't need or want to stick ID's on everything unless you need to target it specifically (e.g. with Javascript). Use classes or just a straight-up div.
From its early days CSS was designed to be markup agnostic so it can be used with any markup language producing tree alike DOM structures (SVG for example). Any tag that comply to name token production is perfectly valid in CSS. So your question is rather about HTML than CSS itself.
Elements with custom tags are supported by HTML5 specification. HTML5 standardize the way how unknown elements must be parsed in the DOM. So HTML5 is the first HTML specification that enables custom elements strictly speaking. You just need to use HTML5 doctype <!DOCTYPE html> in your document.
As of custom tag names themselves...
This document http://www.w3.org/TR/custom-elements/ recommends custom tags you choose to contain at least one '-' (dash) symbol. This way they will not conflict with future HTML elements. Therefore you'd better change your doc to something like this:
<style>
so-cool {
color:blue;
}
</style>
<body>
<so-cool>HELLO</so-cool>
</body>
Surprisingly, nobody (including my past self) mentioned accessibility. Another reason that using valid tags instead of custom ones is for compatibility with the greatest amount of software, including screen-readers and other tools that people need for accessibility purposes. Moreover, accessibility laws like WAI require making accessible websites, which generally means requiring them to use valid markup.
Apparently nobody mentioned it, so I will.
This is a by-product of browser wars.
Back in the 1990’s when the Internet was first starting to go mainstream, competition incrased in the browser market. To stay competitive and draw users, some browsers (most notably Internet Explorer) tried to be helpful and “user-friendly” by attempting to figure out what page designers meant and thus allowed markup that are incorrect (e.g., <b><i>foobar</b></i> would correctly render as bold-italics).
This made sense to some degree because if one browser kept complaining about syntax errors while another ate anything you threw at it and spit out a (more-or-less) correct result, then people would naturally flock to the latter.
While many thought the browser wars were over, a new war between browser vendors has reignited in the past few years since Chrome was released, Apple started growing again and pushing Safari, and IE lost its dominance. (You could call it a “cold war” due to the perceived cooperation and support of standards by browser vendors.) Therefore, it is not a surprise that even contemporary browsers which supposedly conform strictly to web standards actually try to be “clever” and allow standard-breaking behavior such as this in order to try to gain an advantage as before.
Unfortunately, this permissive behavior led to a massive (some might even say cancerous) growth of poorly marked up webpages. Because IE was the most lenient and popular browser, and due to Microsoft’s continued flouting of standards, IE became infamous for encouraging and promoting bad design and propagating and perpetuating broken pages.
You may be able to get away with using quirks and exploits like that on some browsers for now, but other than the occasional puzzle or game or something, you should always stick to web standards when creating web pages and sites to ensure they display correctly and avoid them becoming broken (possibly completely ignored) with a browser update.
While browsers will generally relate CSS to HTML tags regardless of whether or not they are valid, you should ABSOLUTELY NOT do this.
There is technically nothing wrong with this from a CSS perspective. However, using made up tags is something you should NEVER do in HTML.
HTML is a markup language, which means that each tag corresponds to a specific type of information.
Your made up tags don't correspond to any type of information. This will create problems from web crawlers, such as Google.
Read more information on the importance of correct markup.
Edit
Divs refer to groups of multiple related elements, meant to be displayed in block form and can be manipulated as such.
Spans refer to elements that are to be styled differenly than the context they are currently in and are meant to be displayed inline, not as a block. An example is if a few words in a sentence needs to be all caps.
Custom tags do not correlate to any standards and thus span/div should be used with class/ID properties instead.
There are very specific exemptions to this, such as Angular JS
Although CSS has a thing called a "tag selector," it doesn't actually know what a tag is. That's left for the document's language to define. CSS was designed to be used not just with HTML, but also with XML, where (assuming you're not using a DTD or other validation scheme) the tags can be just about anything. You could use it with other languages too, though you would need to come up with your own semantics for exactly what things like "tags" and "attributes" correspond to.
Browsers generally apply CSS to unknown tags in HTML, because this is considered better than breaking completely: at least they can display something. But it is very bad practice to use "fake" tags deliberately. One reason for this is that new tags do get defined from time to time, and if one is defined that looks sort of like your fake tag but doesn't quite work the same way, that can cause problems with your site on new browsers.
Why does CSS work with fake elements? Because it doesn't hurt anyone because you're not supposed to use them anyways.
Why doesn't my professor want me to use made-up elements? Because if that element is defined by a specification in the future your element will have an unpredictable behavior.
Also, why didn't he know that made-up elements exist and work with CSS. Are they uncommon? Because he, like most other web developers, understand that we shouldn't use things that might break randomly in the future.
As far as I know, it is perfectly legal to make up tag names in HTML5, and they work like normal with CSS styling and nesting and all.
Of course my arbitrary tag names will make no difference to the browser, which does not understand them, but it goes very far in making my code more readable, which makes it easier to maintain.
So why should I NOT use arbitrary tag names on my pages? Will it affect SEO? Will it break anything?
IMPORTANT EDIT: Older browsers DO NOT choke on unsupported tags when used with http://ejohn.org/blog/html5-shiv/
UPDATE
The original answer is quite old and was created before web components existed (although they had been discussed since 2011). The rules on custom elements have changed quite a bit. The W3C now has a whole standard for custom elements
Custom elements provide a way for authors to build their own fully-featured DOM elements. Although authors could always use non-standard elements in their documents, with application-specific behaviour added after the fact by scripting or similar, such elements have historically been non-conforming and not very functional. By defining a custom element, authors can inform the parser how to properly construct an element and how elements of that class should react to changes.
In other words, the W3C is now fine with you using custom elements (or arbitrary tag names as the question calls them). However, there are quite a few rules surrounding them and their use so you should refer to the linked documentation.
Original answer
Older browsers choke on tags they don't recognize. There is nothing that guarantees that your custom tags will work as you expect in any browser
There is no semantic meaning applied to your tags since this meaning only exists in the agreed upon tags in the spec. Whether or not this affects SEO is unknown, but anything that parses the HTML may have trouble if it runs into a tag it doesn't recognize including screen readers or web scrapers
You may not get the help you need from the browser if your tags are improperly nested
In the same vein, some browsers may screw up rendering and be different from one another. For example, <p><ul></ul></p> is parsed in Chrome as <p></p><ul></ul>. There is no guarantee how your tags will be treated since they have no agreed upon content model
The W3C doesn't want you to do it.
Authors must not use elements, attributes, or attribute values that are not permitted by this specification or other applicable specifications, as doing so makes it significantly harder for the language to be extended in the future.
If you really need to use custom tags for this purpose, you can use a template language of some sort such as XSLT and create a stylesheet that transforms it into valid HTML
This subject turned into a heated discussion at the office, so I'm interested to learn what you think.
We are working on a web app that only targets some specific browsers. These browsers presently include different flavors Opera 9 and Mozilla 1.7.12. In the future we likely also have to support Opera 10 and different flavors of WebKit. But it's very unlikely we are ever going to have to deal with any version of IE.
Our web app declares HTML 4.0 strict in it's doctype.
Recently, I proposed as a solution to a specific problem to use custom attributes in the HTML. I proposed something that would look like this:
<span translationkey="someKey">...</span>
Since this is not valid HTML 4, it did not go down well with our HTML guys, and we got into an argument.
My question is this: What - if any - are the risks of using custom attributes? I know the page won't validate, but don't all browsers just ignore attributes they do not know? Or is it conceivable that some browsers will change to "quirks mode" and render the page as if it was something other than strict HTML 4.0?
Update:
Hilited the actual question posed.
There are no browser limitations/risks. Only the w3 validator will bark, but barking dogs doesn't bite.
The w3 spec says the following:
If a user agent encounters an attribute it does not recognize, it
should ignore the entire attribute
specification (i.e., the attribute and
its value).
IE will also not render in quirks mode or so as some may think. It will only do that on invalid/forced doctypes, not on invalid attributes.
However, keep in mind that some Javascript libraries/frameworks will "invisibly" add/use custom HTML attributes in the DOM tree, such as several jQuery plugins do. This way you may risk collisions in attributes because it "by a coincidence" uses an attribute with the same name as you do for your own purposes. Sadly this is often poorly or even not documented at all.
HTML 5 allows custom attributes using a 'data-' prefix, see http://ejohn.org/blog/html-5-data-attributes/
If its a goal to maintain valid html4.0 strict, then it doesn't matter why you want to put in custom attributes, you are breaking the goal.
I think the question you need to be asking, is why do you need to break 4.0 strict to get the functionality you want: Anything that you could use a custom attribute for you, you could use a in an existing attribute:
<span translationkey="someKey">...</span>
could be:
<span class="Translationkey#someKey">...</span>
it will be some extra cycles to parse all the class information, but so long as you don't put any css info on that class, it doesn't change display, doesn't put you in quirks mode, and doesn't get you in fights at work.
Duplicate try this thread though: Is it alright to add custom Html attributes?
Also look at this: Non-Standard Attributes on HTML Tags. Good Thing? Bad Thing? Your Thoughts?
Or is it conceivable that some browsers will change to "quirks mode" and render the page as if it was something other than strict HTML 4.0?
No, bad attributes will not force a rendering mode change.
If you don't care about validation do what you like, but validation is a useful tool for detecting simple mistakes that can otherwise have you chasing around debugging. Given that there are many other perfectly good alternatives for passing data to JavaScript I prefer to use one of those, rather than forgo validation.
Plus, when you add an arbitrary attribute you are effectively playing in a global namespace. There's no guarantee that some future browser or standard won't decide to use the name ‘translationkey’ for some new feature that'll trip your script up. So if you must add attributes, give them a name that's obscure and likely to be unique, or just use the HTML5 data- prefix already.
If the page is declared to be HTML 4 strict, then it should not add attributes that are not used in that HTML specifies. Differently, it is not clear what the browsers would behave.
As already reported, a way to add additional attributes is to add them as classes, even if that has some limitations.
(Copying my answer from a duplicate question)
Answers which say custom attributes won't validate are incorrect.
Custom attributes will validate.
Custom tags will validate too, as long as the custom tags are lowercase and hyphenated.
Try this in any validator. It will validate.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Custom Test</title>
</head>
<body>
<dog-cat PIANO="yellow">test</dog-cat>
</body>
</html>
Some validators:
https://appdevtools.com/html-validator
https://www.freeformatter.com/html-validator.html
https://validator.w3.org/nu/
But is it safe? Will it break later?
Custom Tags
No hyphenated tags exist. For that reason, I believe that W3C will never add a hyphenated tag. It's very likely a custom hyphenated tag will never see a conflict. If you use a weird prefix in your custom tags, even less likely. Eg.<johny-Calendar>.
Custom Attributes
There are hyphenated HTML attributes. But the HTML spec promises never to use an attribute starting with data-. So data-myattrib is guaranteed to be safe.
I believe that W3C will never introduce any attribute that starts with johny- or piano-. As long as your prefix is weird, you'll never see a conflict.
It's worth considering that the first version of HTML was written in 1993. Now, thirty years later, all browsers still support custom tags and attributes, and validators validate them.
<jw-post jw-author="Johny Why" jw-date="12/2/2022">
Do you know of any differences in handling HTML tags/properties in different browsers? For example, I once saw a page with a input tag with a maxlength field set to "2o". Firefox and Opera ignore the "o", and set the max length to 2, while Internet Explorer ignores the field altogether. Do you know of any more?
(Note: seeing as this will probably be a list, it would be great if the general name of the difference was in bold text, like: Different erratic value handling in tag properties)
Bug Lists
Web developers have already compiled some pretty comprehensive lists; I think it's better to compile a list of resources than to duplicate those lists.
http://www.positioniseverything.net/
http://www.richinstyle.com/bugs/table.html
http://www.quirksmode.org/ (as mentioned by Kristopher Johnson)
Javascript
I agree with Craig - it's best to program Javascript using a library that handles differences between browsers (as well as simplify things like namespacing, AJAX event handling, and context). Here's the jump to Craig's answer (on this page).
CSS Resets
CSS Resets can really simplify web development. They override settings which vary slightly between browsers to give you a more common starting point. I like Yahoo's YUI Reset CSS.
Check out http://www.quirksmode.org/
If you are programming in javascript the best advice I can give is to use a javascript library instead of trying to roll your own. The libraries are well tested, and the corner cases are more likely to have been encountered.
Scriptalicious - http://script.aculo.us/
jQuery - http://jquery.com/
Microsoft AJAX - http://www.asp.net/ajax/
Dojo - http://dojotoolkit.org/
Prototype - http://www.prototypejs.org/
YUI - http://developer.yahoo.com/yui/
Do you know of any differences in handling HTML tags/properties in different browsers
Is this question asking for information on all differences, including DOM and CSS? Bit of a big topic. I thought the OP was asking about HTML behaviour specifically, not all this other stuff...
The one that really annoys me is IE's broken document.getElementById javascript function - in most browsers this will give you something that has the id you specify, IE is happy to give you something that has the value in the name attribute, even if there is something later in the document with the id you asked for.
I once saw a page with a input tag
with a maxlength field set to "2o".
In this specific case, you're talking about invalid code. The maxlength attribute can't contain letters, only numbers.
What browsers do with invalid code varies a great deal, as you can see for yourself.
If you're really asking "what do all the different browsers do when faced with HTML code that, for any one of an infinite number of reasons, is broken?", that way lies madness.
We can reduce the problem space a great deal by using valid code.
So, use valid HTML. Then you are left with two main problem areas:
browser bugs -- how the browser follows the HTML standard and what it does wrong
differences in browser defaults, like the amount of padding/margin it gives to the body
Inconsistent parsing of XHTML in HTML mode
HTML parsers are not designed to handle XML.
If an XHTML document is served as "text/html“ and the compatibilities guidelines are not followed you can get unexpected results.
Empty tags is one possible source of problems. <tag/> and <tag></tag> are equivalent in XML. However the HTML parser can interpret them in two ways.
For instance Opera and IE treat <br></br> as two <br> but Firefox and WebKit treat <br></br> as one <br>.