Recently I found that there is, possibly, a new way of commenting in HTML5.
Instead of the typical <!-- --> multi-line commenting I've read about, I thought I noticed that my IDE made a regular <!div > commented out. So I tested it out, and to my surprise Chrome had commented out that tag. It only commented out the tag and not the contents of the div, so I had to comment out the closer <!/div> to avoid closing other divs.
I tested another and it appears that generally putting an exclamation marker in front of the opening of any tag, this symbol <, makes that tag commented out.
Is this actually new? Is it bad practice? It is actually very convenient, but is it practical yet (if not new)?
Extra details:
Although a syntax error or misinterpretations of this particular syntax is a good reason, how come Chrome actually renders them as full comments?
The code is written as:
<!div displayed> some text here that is still displayed <!/div>
And then it is rendered as:
<!--div displayed--> some text here that is still displayed <!--/div-->
There is no new standard for comments in HTML5. The only valid comment syntax is still <!-- -->. From section 8.1.6 of W3C HTML5:
Comments must start with the four character sequence U+003C LESS-THAN SIGN, U+0021 EXCLAMATION MARK, U+002D HYPHEN-MINUS, U+002D HYPHEN-MINUS (<!--).
The <! syntax originates in SGML DTD markup, which is not part of HTML5. In HTML5, it is reserved for comments, CDATA sections, and the DOCTYPE declaration. Therefore whether this alternative is bad practice depends on whether you consider the use of (or worse, the dependence on) obsolete markup to be bad practice.
Validator.nu calls what you have a "Bogus comment." — which means that it's treated like a comment even though it's not a valid comment. This is presumably for backward compatibility with pre-HTML5, which was SGML-based, and had markup declarations that took the form <!FOO>, so I wouldn't call this new. The reason they're treated like comments is because SGML markup declarations were special declarations not meant to be rendered, but since they are meaningless in HTML5 (with the above exceptions), as far as the HTML5 DOM is concerned they are nothing more than comments.
The following steps within section 8.2.4 lead to this conclusion, which Chrome appears to be following to the letter:
8.2.4.1 Data state:
Consume the next input character:
"<" (U+003C)
Switch to the tag open state.
8.2.4.8 Tag open state:
Consume the next input character:
"!" (U+0021)
Switch to the markup declaration open state.
8.2.4.45 Markup declaration open state:
If the next two characters are both "-" (U+002D) characters, consume those two characters, create a comment token whose data is the empty string, and switch to the comment start state.
Otherwise, if the next seven characters are an ASCII case-insensitive match for the word "DOCTYPE", then consume those characters and switch to the DOCTYPE state.
Otherwise, if there is an adjusted current node and it is not an element in the HTML namespace and the next seven characters are a case-sensitive match for the string "[CDATA[" (the five uppercase letters "CDATA" with a U+005B LEFT SQUARE BRACKET character before and after), then consume those characters and switch to the CDATA section state.
Otherwise, this is a parse error. Switch to the bogus comment state. The next character that is consumed, if any, is the first character that will be in the comment.
Notice that it says to switch to the comment start state only if the sequence of characters encountered is <!--, otherwise it's a bogus comment. This reflects what is stated in section 8.1.6 above.
8.2.4.44 Bogus comment state:
Consume every character up to and including the first ">" (U+003E) character or the end of the file (EOF), whichever comes first. Emit a comment token whose data is the concatenation of all the characters starting from and including the character that caused the state machine to switch into the bogus comment state, up to and including the character immediately before the last consumed character (i.e. up to the character just before the U+003E or EOF character), but with any U+0000 NULL characters replaced by U+FFFD REPLACEMENT CHARACTER characters. (If the comment was started by the end of the file (EOF), the token is empty. Similarly, the token is empty if it was generated by the string "<!>".)
In plain English, this turns <!div displayed> into <!--div displayed--> and <!/div> into <!--/div-->, exactly as described in the question.
On a final note, you can probably expect other HTML5-compliant parsers to behave the same as Chrome.
I don't think this is a good habit to take since <! stands for markup declarations like <!DOCTYPE. Thus you think it's commented (well... browser will try to interpret it).
Even if it doesn't appear, this seems not to be the correct syntax for commenting HTML code.
Related
I'd like to remove more unnecessary bytes from my output, and it seems it's acceptable (in practice) to strip what can add up to quite a lot of whitespace from HTML markup by omitting/collapsing the gaps between DOM element attributes.
Although I've tested and researched (a little in both cases), I'm wondering how safe it would be?
I tested in Chrome (43.0.2357.65 m), IE (11.0.9600.17801), FF (38.0.1) and Safari (5.1.7 (blah-di-blah)) and they didn't seem to mind, and couldn't find anything specific in The Specs about whitespace between attributes.
w3.org's Validator complains, which is a strong indication that this is not safe and shouldn't be expected to work, but (there's always a "but") it's possible the requirement for a space is only strict when no quotes are present (for obvious reasons).
Also (snippy but poignant): their SSL is "out of date" which doesn't inspire confidence in their opinion.
I noted also that someone's HTML compressor could (when enabled) strip quotes around attribute values where those values had no whitespace within them (e.g. id), which implies that at least most if not all HTML parsing is focussed on the text either side of the equals signs (except with booleans of course), and where quotes are in use, they'd be considered the prioritized delimiter.
So, would:
<!DOCTYPE html><html><body>
Yabba Dabba Doo!
</body></html>
▲ that ever go wrong, and if so, under which conditions?
What other reasons could there be to maintain this whitespace in production output (code "readability" is a non issue in this case)?
Update (since finding an answer):
Although I basically answered my own question insofar that there is a specification governing whether there should be a space between attributes, I still wonder if omitting them when using quoted values can be considered practically safe, and would appreciate feedback on this point.
Considering how often spaces may be omitted by accident in production HTML, and that the browsers I tested don't seem to mind when they are, I assume it would be very rare if ever that a browser failed to handle documents with these spaces omitted.
Although it's sensible to follow the specs in pretty much all situations, might this be one time cheating a bit could be acceptable?
After all - if we can magically save several hundred bytes without affecting the quality of the output, why not?
There is a specification (after all)
It turns out I should have looked harder. My bad.
According to these specs:
If an attribute using the empty attribute syntax is to be followed by another attribute, then there must be a space character separating the two.
and
If an attribute using the unquoted attribute syntax is to be followed by another attribute or by the optional U+002F SOLIDUS character (/) allowed in step 6 of the start tag syntax above, then there must be a space character separating the two.
and
If an attribute using the single-quoted attribute syntax is to be followed by another attribute, then there must be a space character separating the two.
and
If an attribute using the double-quoted attribute syntax is to be followed by another attribute, then there must be a space character separating the two.
Which unless I am mistaken (again), means there must always be spaces between attributes.
You could try online HTML minifiers like http://www.whak.ca/minify/HTML.htm or http://www.scriptcompress.com/minify-HTML.htm (search google for more) and find little things they change for hints to what can be taken out yet still render the HTML code.
On the first link your code:
<!DOCTYPE html><html><body>
Yabba Dabba Doo!
</body></html>
Turns into:
<!DOCTYPE html><html><body>Yabba Dabba Doo!
saving you 18 bytes already...
Every page on the plosone website seems to have:
# wombat urls
ß
Before any actual HTML code begins. Why would this be? I've seen a tick being placed in the title before to check (force?) UTF8 support, but it seems weird to have this whole phrase there, and even weirder to place it outside of the HTML code...
It is likely a mistake
Whether intentional or by accident, the resulting code is invalid. The code snippet you reference is likely an unintentional relic from a server-side script.
According to the W3C, documents must consist of the following parts, in the given order:
Optionally, a single "BOM" (U+FEFF) character.
Any number of comments and space characters.
A DOCTYPE.
Any number of comments and space characters.
The root element, in the form of an html element.
Any number of comments and space characters.
The code snippet is not a "BOM" (byte-order mark) character, HTML comment, nor combination of space characters; so it is invalid.
It is also worth noting that while most modern browsers can cope with this invalid code snippet (by ignoring it), it would trigger some versions of Internet Explorer to render the page content in "Quirks Mode". See this answer to the question "Can comments appear before the DOCTYPE declaration?"
In HTML 5 specification the parser and the specification state that the element name can be everything starting with a letter and followed by alpha-numeric characters.
Now the question is what happens if I introduce additional elements not part of the specification but valid in terms of compliance to the specified syntax.
What do all those browsers do when they encounter elements with custom yet unknown name? Does those elements got treaten like any element or are they left out, stripped out or replaced?
How do for instance do HTML5 editor behave?
Is there anything in the specifications I have overlooked regarding valid element tag names?
[Update]
The specification was missleading here since it states the alpha-numerical character of the HTML element names. While reading a HTML 5 Specification, I missconcluded that this is true for all element names.
That is appearently wrong. In the parser section it states that a element name must only start with an ASCII letter and after that letter everthing except:
"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE
"/" (U+002F)
">" (U+003E)
U+0000 NULL
EOF
Beside those mentioned characters which require special treatment involving errors or ending the tag name all other possible characters seam to be allowed.
Anything else
--> Append the current input character to the current tag token's tag name.
From my field test also additional uni-code letters are allowed for the first letter by several parsers (at least they are graceful with those).
[/Update]
In HTML 5 specification the parser and the specification state that
the element name can be everything starting with a letter and followed
by alpha-numeric characters.
Incorrect. The specification states that the element name must be one of the names explicitly listed in that document, or in another applicable specification. These include but are not limited to SVG and MathML.
The specification also includes a processing specification for consumers of HTML, such as browsers. This doesn't describe what's "allowed", it describes what those consumers should do with each character of the document regardless of whether it contains things that are allowed or not allowed.
Now the question is what happens if I introduce additional elements
not part of the specification but valid in terms of compliance to the
specified syntax.
The above rules are followed. The "specified syntax" is irrelevant. The specification describes what the consumer should do for any input stream of characters.
What do all those browsers do when they encounter elements with custom
yet unknown name? Does those elements got treated like any element or
are they left out, stripped out or replaced?
They are treated as elements in the http://www.w3.org/1999/xhtml namespace which implement the HTMLUnknownElement interface.
How for instance do HTML5 editor behave?
If they are HTML5 compliant they will behave the same way when reading in the HTML.
Is there anything in the specifications I have overlooked regarding
valid element tag names?
See the first paragraph above. Also the Custom Elements spec which makes any element name starting with an ASCII letter and containing a hyphen to be considered valid. It is unclear whether that specification is currently an "HTML5 applicable specification" but if not, it will very probably be one soon.
i have been working with HTML for some time, recently a question popped in my mind that when ever give tags
<input type = "text" name="c-id" />
<input type =text name=c-id />
with or without quotes there is no difference, when working with V-Studio, this IDE doesn't put quotes automatically, but while working with the Dreamweaver if i remember correctly it does put the quotes automatically...
what i want is to know is that at what point\s the absence of quotes makes a difference or creates a problem, and what is the best practice, always go quoted or quotes-less...
p.s. there is very good chance this question has been asked before but didn't show up in the search
It makes a difference when the attribute value contains a space, linefeed, formfeed, or tab character, because the attribute value will end at any one of those characters if the value is not wrapped in quotes.
From an IDE point of view, it's little more than a matter of preference whether to add the quotes or not when the attribute value does not contain one of those whitespace characters.
In XHTML syntax, the quotation marks are always required (but you can alternatively use single quotes, i.e. 'apostrophes'). This does not matter if the page is served with text/html content type, as it almost always is: browsers will parse it as sloppy HTML, not as real XHTML. But if the page is served with an XML content type, or if it is opened in a program that expects XHTML or other XML, then lack of quotation marks causes Draconian error processing: only an error message is shown, not the content at all.
In the example case, XHTML syntax is used otherwise: the “/” before closing “>” belongs to XHTML, not HTML. It makes little point to write XHTML in that respect but not consistently.
In HTML syntax, the formal rules depend on HTML version. HTML5 is much more permissive than HTML 4.01. For example, <a href=/foo/bar/ title=What???> is valid HTML5 but not valid HTML 4.01. This mostly matters in validation and depend on which version of HTML you wish to validate against (i.e., which one do you mostly try to comply with). In this issue, HTML5 reflects browser practices: browsers have long been more permissive than HTML 4.01.
By HTML5 rules, the quotes (though always allowed) are needed only when the attribute value contains any of the following characters: space, tab, line feed, form feed, carriage return, quotation mark ("), apostrophe ('), equals sign (=), less-than sign (<), greater-than sign (>), or grave (`), or is empty.
There are also opinions as well as coding style guides and other recommendations on the matter. Most people who think about such issues apparently favor the “safe” way of always putting quotes around attribute values. Some people think the quotes improve readability of code; others think they reduce it.
Not putting your values into quotes may cause some problems, ie you can't provide multiple css classes for your elements. Browser will only understand the first value, and the page won't pass validation.
You should always put your values into quotes. The markup is easier to read and you won't run into some unexpected browser behaviour.
For me, any practice that improves code/markup (re-)readability is worth making a habit, and having quotes on the attribute values visibly separates one property from another (readability again).
On the technical side, some attribute values have spaces and the whole tag might not work without the quotes on these. Those that come to mind: img's alt, multiple class values, title (for tooltips), an href with a url that has the equal sign (without quotes, the first equal in href= is doomed), etc.
And here is the almost same question in SO.
--
My thanks to Alohci for enlightening me. I can't up vote yet.
When outputting HTML, there are several different places where text can be interpreted as control characters rather than as text literals. For example, in "regular" text (that is, outside any element markup):
<div>This is regular text</div>
As well as within the values of attributes:
<input value="this is value text">
And, I believe, within HTML comments:
<!-- This text here might be programmatically generated
and could, in theory, contain the double-hyphen character
sequence, which is verboten inside comments -->
Each of these three kinds of text has different rules for how it must be escaped in order to be treated as non-markup. So my first question is, are there any other contexts in HTML in which characters can be interpreted as markup/control characters? The above contexts clearly have different rules about what needs to be escaped.
The second question is, what are the canonical, globally-safe lists of characters (for each context) that need to be escaped to ensure that any embedded text is treated as non-markup? For example, in theory you only need to escape ' and " in attribute values, since within an attribute value only the closing-delimiter character (' or " depending on which delimiter the attribute value started with) would have control meaning. Similarly, within "regular" text only < and & have control meaning. (I realize that not all HTML parsers are identical. I'm mostly interested in what is the minimum set of characters that need escaping in order to appease a spec-conforming parser.)
Tangentially: The following text will throw errors as HTML 4.01 Strict:
foo
Specifically, it says that it doesn't know what the entity "&y" is supposed to be. If you put a space after the &, however, it validates just fine. But if you're generating this on the fly, you're probably not going to want to check whether each use of & will cause a validation error, and instead just escape all & inside attribute values.
<div>This is regular text</div>
Text content: & must be escaped. < must be escaped.
If producing a document in a non-UTF encoding, characters that do not fit inside the chosen encoding must be escaped.
In XHTML (and XML in general), the sequence ]]> must not occur in text content, so in that specific case one of the characters in that sequence must be escaped, traditionally the >. For consistency, the Canonical XML specification chooses to escape > every time in text content, which is not a bad strategy for an escaping function, though you can certainly skip it for hand-authoring.
<input value="this is value text">
Attribute values: & must be escaped. The attribute value delimiter " or ' must be escaped. If no attribute value delimiter is used (don't do that) no escape is possible.
Canonical XML always chooses " as the delimiter and therefore escapes it. The > character does not need to be escaped in attribute values and Canonical XML does not. The HTML4 spec suggested encoding > anyway for backwards compatibility, but this affects only a few truly ancient and dreadful browsers that no-one remembers now; you can ignore that.
In XHTML < must be escaped. Whilst you can get away with not escaping it in HTML4, it's not a good idea.
To include tabs, CR or LF in attribute values (without them being turned into plain spaces by the attribute value normalisation algorithm) you must encode them as character references.
For both text content and attribute values: in XHTML under XML 1.1, you must escape the Restricted Characters, which are the Delete character and C0 and C1 control codes, minus tab, CR, LF and NEL. In total, [\x01-\x08\x0B\x0C\x0E-\x1F\x7F-\x84\x86-\x9F]. The null character may not be included at all even escaped in XML 1.1. Outside XML 1.1 you can't use any of these characters at all, nor is there a good reason you'd ever want to.
<!-- This text here might be programmatically generated
and could, in theory, contain the double-hyphen character
sequence, which is verboten inside comments -->
Yes, but since there is no escaping possible inside comments, there is nothing you can do about it. If you write <!-- < -->, it literally means a comment containing “ampersand-letter l-letter t-semicolon” and will be reflected as such in the DOM or other infoset. A comment containing -- simply cannot be serialised at all.
<![CDATA[ sections and <?pis in XML also cannot use escaping. The traditional solution to serialise a CDATA section including a ]]> sequence is to split that sequence over two CDATA sections so it doesn't occur together. You can't serialise it in a single CDATA section, and you can't serialise a PI with ?> in the data.
CDATA-elements like <script> and <style> in HTML (not XHTML) may not contain the </ (ETAGO) sequence as this would end the element early and then error if not followed by the end-tag-name. Since no escaping is possible within CDATA-elements, this sequence must be avoided and worked around (eg. by turning document.write('</p>') into document.write('<\/p>');. (You see a lot of more complicated silly strategies to get around this one, like calling unescape on a JS-%-encoded string; even often '</scr'+'ipt>' which is still quite invalid.)
There is one more context in HTML and XML where different rules apply, and that's in the DTD (including the internal subset in the DOCTYPE declaration, if you have one), where the % character has Special Powers and would need to be escaped to be used literally. But as an HTML document author it is highly unlikely you would ever need to go anywhere near that whole mess.
The following text will throw errors as HTML 4.01 Strict:
foo
Yes, and it's just as much an error in Transitional.
If you put a space after the &, however, it validates just fine.
Yes, under SGML rules anything but [A-Za-z] and # doesn't start parsing as a reference. Not a good idea to rely on this though. (Of course, it's not well-formed in XHTML.)
The above contexts clearly have different rules about what needs to be escaped.
I'm not sure that the different elements have different encoding rules like you say. All the examples you list require the HTML encoding.
E.g.
<h1>Fish & Chips</h1>
<img alt="Awesome picture of Meat Pie & Chips" />
Fish & Chips
The last example includes some URL Encoding for the ampersand too (&) and its at this point things get hairy (sending an ampersand as data, which is why it must be encoded).
So my first question is, are there any other contexts in HTML in which characters can be interpreted as markup/control characters?
Anywhere within the HTML document, if the control characters are not being used as control characters, you should encode them (as a good rule of thumb). Most of the time, its HTML Encoding, & or > etc. Othertimes, when trying to pass these characters via a URL, use URL Encoding %20, %26 etc.
The second question is, what are the canonical, globally-safe lists of characters (for each context) that need to be escaped to ensure that any embedded text is treated as non-markup?
I'd say that the Wikipedia article has a few good comments on it and might be worth a read - also the W3 Schools article I guess is a good point. Most languages have built in functions to prepare text as safe HTML, so it may be worth checking your language of choice (if you are indeed even using any scripting languages and not hand coding the HTML).
Specifically, Wikipedia says: "Characters <, >, " and & are used to delimit tags, attribute values, and character references. Character entity references <, >, " and &, which are predefined in HTML, XML, and SGML, can be used instead for literal representations of the characters."
For URL Encoding, this article seems a good starting point.
Closing thoughts as I've already rambled a bit: This is all excluding the thoughts of XML / XHTML which brings a whole other ballgame to the court and its requirement that pretty much the world and its dog needs to be encoded. If you are using a scripting language and writing out a variable via that, I'm pretty sure it'll be easier to find the built in function, or download a library that'll do this for you. :) I hope this answer was scoped ok and didn't miss the point or question or come across in the wrong tone. :)
If you are looking for the best practices to escape characters in web browsers (including HTML, JavaScript and style sheets), the XSS prevention cheat sheet by Michael Coates is probably what you're looking for. It includes a description of the different interpretation contexts, tables indicating how to encode characters in each context and code samples (using ESAPI).
http://www.owasp.org/index.php/XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet
Beware that <script> followed by <!-- followed by <script> again, enters double-escaped state, in which you probably never want to be, so ideally you should escape < with "\u003C" within your script's strings (and regexps) to not trigger it accidentally.
You can read more about it here http://qbolec-memdump.blogspot.com/2013/11/script-tag-content-madness.html
If you are this concerned about the validity of the final HTML, you might consider constructing the HTML via a DOM, versus as text.
You don't say what environment you are targeting.