After keeping in mind that HTML has both an SGML and XML serialisations, which are just encodings for a parser to "explode" into a DOM, I'm wondering whether there are other serialisations for HTML. A JSON serialisation? If so, are there any parsers for these alternative serialisations?
Useful serializations? No. Obviously, people can make up their own serializations, and given the size of the Web, there's probably a number of them. However, none of those are supported widely. For example, there's the format used by html5lib tests, which is used for testing HTML5 parsing implementations.
(Also note that what you call the SGML serialization isn't really SGML at all, as noted in HTML5.)
Related
Here https://en.wikipedia.org/wiki/Tag_soup#HTML5 it's written:
HTML5 aims to be the most complete solution to the problem of tag soup
thus far while remaining as backwards- and forwards-compatible as
possible. By contrast to XHTML, which departs from backwards
compatibility and takes the approach that parsers should become less
tolerant of badly formed markup, HTML5 acknowledges that badly formed
HTML code already exists in large quantities and will probably
continue to be used, and takes the view that the specification should
be expanded to ensure maximum compatibility with such code.
Thus, the HTML 5 specification has altered its definition of HTML
syntax both to accommodate common syntax in use today, and to
explicitly describe exactly how "badly formed code" should be treated
by the parser. The handling of badly formed code now has a place in
the specification itself, hopefully reducing the need for future HTML
parsers to implement additional, out-of-specification measures for
dealing with code that it does not recognize.
Do I understand right then that a html5 parser should parse older html pages (like html 2.0 or html 4) correctly? I need a html parser that would parse normally most of internet pages. So I found Google Gumbo: https://github.com/google/gumbo-parser. It's written there that it's HTML5 parser. Will it suit me then to parse not html5 web pages?
Yes, that's one of the main differences between HTML5 and XHTML. You should be able to parse any HTML page with a HTML5 parser.
So, yeah, is HTML a particular application of XML? Like, instead of user-customizable tags, "hard coded" fixed tags decided by the W3C and interpreted by navigators? Or are them totally different things?
Also, in which case is XML better than a database to transfer information inside a Web application? (I was thinking, saving users information or things like that may do better with XML documents than with a database).
Here's a history of HTML
...The HTML that Tim invented was strongly based on SGML (Standard Generalized Mark-up Language), an internationally agreed upon method for marking up text into structural units such as paragraphs, headings, list items and so on. SGML could be implemented on any machine. The idea was that the language was independent of the formatter (the browser or other viewing software) which actually displayed the text on the screen. The use of pairs of tags such as and is taken directly from SGML, which does exactly the same. The SGML elements used in Tim's HTML included P (paragraph); H1 through H6 (heading level 1 through heading level 6); OL (ordered lists); UL (unordered lists); LI (list items) and various others. What SGML does not include, of course, are hypertext links: the idea of using the anchor element with the HREF attribute was purely Tim's invention, as was the now-famous `www.name.name' format for addressing machines on the Web....
And in no case is XML "better" than a database (are cakes better than ovens?). XML isn't for storing data, it's for transfering it. Unless the data is absolutely minimal, you have to find some other way to store it. Opening static XML files on the file system over and over as you save and read data is a terrible way to go about it.
So, yeah, is HTML a particular application of XML?
No.
HTML 4 is an application of SGML, but most parsers for it do not treat it as such.
XHTML is an application of XML, but it is usually served as text/html instead of application/xhtml+xml and so is treated like HTML.
HTML 5 is not an application of either SGML or XML (except in its XML serialisation) and has its own parsing rules.
Also, in which case is XML better than a database to transfer information inside a Web application?
XML is a good basis for a data exchange format. It is not a good basis for storing data in order to search it (which is what happens "inside" most web applications)
HTML and XML both come from SGML, hence their similarities. But XML is a strict grammar (no predefined tag names), while HTML is both a not very strict grammar and a vocabulary (tag names). There is an HTML variant which strictly complies with XML rules : XHTML.
As for using XML as a database, it is possible under certain circumstances. But it really depends on your architecture, language, volumetry and lots of other considerations. I suggest you open a new question with more details for this.
XHTML is a reformulation of HTML as XML app.
You can invent your own tags. I don't think HTML5 has a doctype for that though. You can create them with JavaScript and initalize/style then with CSS like any other element.
instead of using XML, spit out JSON, seriously, do this.
if you are worried about using your db, think about switching to couchdb or nosql. they're ripe for JSON.
don't get me wrong, your thought process isn't wrong, you can do that. i've seen it done rather well. but most people don't get it right. and seriously, JSON is your friend.
For the differences between HTML & XML see:
http://www.w3schools.com/xml/xml_whatis.asp
XML is primarily used for transfering data, not storing it. A database will generally give you much more flexibility in querying the data.
HTML allows things that XML doesn't allow, like omitting end tags, omitting the quotes around attribute values, and using upper-case and lower-case interchangeably. So HTML is not just another XML vocabulary.
XHTML, however, was an attempt to reformulate HTML as an XML vocabulary.
Reading some related questions made me think about the theoretical nature of HTML.
I'm not talking about XHTML-like code here. I'm talking about stuff like this crazy piece of markup, which is perfectly valid HTML(!)
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html<head>
<title//
<p ltr<span id=p></span</p>
</>
So given the enormous complexity that SGML injects here, is HTML a context-free language? Is it a formal language anyway? With a grammar?
What about HTML5?
I'm new to the concept of formal languages, so please bear with me. And yes, I have read the wikipedia article ;)
Context Free is a concept from language theory that has important implications in parser implementation. A Context Free Language can be described by a Context Free Grammar, which is one in which all rules have a single non-terminal symbol at the left of the arrow:
X→δ
That simple restriction allows X to be substituted by the right-hand side of the rules in which appears on the left without regard to what came before or after. For example, if while deriving or parsing one arrives at:
αXλ
one is sure that
αδλ
is also valid. Examples of non-context-free rules would be:
XY→δ
Xa→δ
aX→δ
Those would require knowing what could be derive arround X to determine if a rule applies, and that leads to non-determinism (what's around X would also like to know what it derives to), which is a no-no in parsing, and in any case we want a language to be well-defined.
The only way to prove that a language is context-free is by proving that there's a context-free grammar for it, which is not an easy task. Most programming languages one comes about are already described by CFGs, so the job is done. But there are other languages, including programming languages, that are described using logic or plain English, so work is required to find if they are context-free.
For HTML, the answer about its context-freedom is yes. SGML is a well defined Context Free Language, and HTML defined on top of it is also a CFL. Parsers and grammars for both languages abound on the Web. At any rate, that there exist LL(k) grammars for valid HTML is enough proof that the language is context-free, because LL is a proven subset of CF.
But the way HTML evolved over the life of the Web forced browsers to treat it as not that well defined. Modern Web browsers will go out of their way to try to render something sensible out of almost anything they find. The grammars they use are not CFGs, and the parsers are far more complex than the ones required for SGML/HTML.
HTML is defined at several levels.
At the lexical level there are the rules for valid characters, identifiers, strings, and so on.
At the next level is XML, which consists of the opening and closing <tags> that define a hierarchical document structure. You can use XML or something XML-like for any purpose, like Apache Ant does for build scripts.
At the next level are the tags that are valid in HTML, and the rules about which tags may be nested within which tags.
At the next level are the rules about which attributes are valid for which tags, languages that can be embedded in HTML like CSS and JavaScript.
Finally, you have the semantic rules about what a given HTML document means.
The syntactic part is defined well enough that it can be verified. The semantic part is much larger than the syntactic one, and is defined in terms of browser actions regarding HTTP, and the Document Object Model (DOM), and how a model should be rendered to the screen.
In the end:
Parsing correct HTML is extremely easy (it's context-free and LL/LR).
Parsing the HTML that actually exists over the Web is difficult.
Implementing the semantics (a browser) over HTML/CSS/DOM is extremely difficult.
Valid HTML is not a context-free language.
First of all, HTML being an application of SGML is fiction for all practical purposes, so analyzing SGML to answer the question is useless. (However, the SGML fiction probably isn't context-free, either.)
It's more useful to look at the actually defined HTML parsing algorithm. It works on two levels: tokenization and tree building. What HTML calls tokenization is a higher-level operation than what is usually called tokenization when talking about parsers. In the case of HTML, tokenization splits a stream of characters into units like start tags, end tags, comments and text. The tokenizer expands character references. Usually, when talking about parsers, you'd probably treat stuff like the less-than sign as "tokens" and would consider character references to consist of tokens instead of being resolved by the tokenizer.
If you consider the process of splitting the input stream into tokens, that level of the HTML language is regular (except for feedback from the tree builder).
However, there are three complications: The first one is that splitting the input stream into tokens is just the first and then there's the tree builder's side that actually cares about the identifiers in the tokens. The second one is that the tree builder feeds back into the tokenizer so that some state transitions made by the tokenizer depend on the state of the tree builder! The third one is that valid documents in the language are defined by rules that apply to the output of the tree builder stage and those rules are complex enough that they can't be fully defined using tree automata (as evidenced by RELAX NG not being expressive enough to describe all the validity constraints).
This isn't an actual proof, but you can probably develop real proofs by working from complications #2 and #3.
Note that the case of invalid documents is not particularly interesting as a question of whether the language is context-free in the sense of there being a context-free grammar that generates all the possible strings with no regard to the parse tree having some intelligible interpretation in terms of the tree that an HTML parser generates. The HTML parser will successfully consume all possible strings, so in that sense, all possible strings are in the "invalid HTML" language.
Edit: Interesting questions left as exercise to the reader:
Is HTML without parse errors but ignoring validity a context-free language?
Is HTML without parse errors and ignoring general validity but with only valid element names allowed a context-free language?
(Complication #2 applies in both cases.)
NO
See Edit Below
It depends.
If you are talking about the subset consisting of only theoretical HTML, then yes.
If you also include real life, working HTML that is accessed and used successfully by millions of people daily on many of the top sites on the internet then NO.
That is what gives HTML flexibility. The parsing engine adds tags, closes tags, and takes care of stuff that a theoretical CFG can't do. If you took automata you might remember that a production rule in a formal grammar cannot be empty (aka epsilon/lambda) on the lhs (left-hand side). Since the parsing engine is basically using knowledge that a formal grammar and automata couldn't have, it isn't restricted by that and the 'grammar' would have epsilon/lambda -> result where the specific epsilon/lambda rule is chosen based on information not available in the grammar.
Since I don't think empty lhs are allowed in any formal grammars, HTML cannot be defined by a formal grammar and is not a formal language at all.
Sure, HTML5 might try to move towards a 'more formal' language description but the likelihood that it becomes a context free language in reality (i.e. strings not matched by the grammar are rejected) is about the likelihood XHTML 2.0 takes the world by storm and replaces HTML altogether (XHTML is the attempt they made to make HTML a formal language...it was rejected en masse due to its fragility).
Noteworthy is the fact that HTML 5 is the FIRST HTML standard to be defined before being implemented! That's right, HTML 1-4 consist of random ideas someone just implemented in a browser, and were collected into standards after the fact based on which features were popularly used and widely implemented. Then they tried XHTML, which totally failed to be adopted. Even 'xhtml' on the web is automatically parsed as HTML under almost every circumstance to prevent stuff from just breaking with a cryptic syntax error. Now you can see how we got here and why it is unlikely to be formalized any time soon.
Lesson: "In theory, there is no difference between theory and practice. In practice, there is." - Yogi Berra
EDIT:
Actually, after reading through the documents it turns out that HTML, even according to the HTML 4.01 specification, doesn't actually conform to SGML. To see for yourself, view the HTML 4.01 Strict document type definition (doctype) at http://www.w3.org/TR/html4/strict.dtd and note the following lines:
The HTML 4.01 specification includes additional
syntactic constraints that cannot be expressed within
the DTDs.
So I would say that it is probably not a CFL due to those features (although it technically it doesn't disprove the hypothesis that there is some possible PDA that accepts HTML 4.01, it does prevent the argument that SGML is a CFL therefore HTML is a CFL).
HTML5 flip-flops, abandoning any implied conformance to SGML, but is presumably describable by a CFG. However it will still provide best-effort parsing not based on a cfg, so IMO the current situation (i.e. language specification is defined formally, with invalid strings still being accepted, parsed and rendered in a best effort fashion) in this regard is unlikely to change drastically for a long, long, long time.
HTML5 is different from previous HTML versions in that it strictly defines the parsing behaviour of code that isn't completely correct. Pre-HTML5 parsers vary and each do their best to 'guess' the intention of the code author.
I've been considering converting my current HTML5 documents to polyglot HTML5 ones. I figure that even if they only ever get served as text/html, the extra checks of writing it XML would help to keep my coding habits tidy and valid.
Is there anything particularly thrilling in the HTML5-only space that would make this an unwise choice?
Secondly, the specs are a bit hazy on how to validate a polyglot document. I assume the basics are:
No errors when run through the W3C Validator as HTML5
No errors when run through an XML parser
But are there any other rules I'm missing?
Thirdly, seeing as it is a polyglot, does anyone know any caveats to serving it as application/xhtml+xml to supporting browsers and text/html to non-supporting ones?
Edit: After a small bit of experimenting I found that entities like break in XHTML5 (no DTD). That XML parser is a bit of a double-edged sword, I guess I've answered my third question already.
Work on defining how to create HTML5 polyglot documents is currently on-going, but see http://dev.w3.org/html5/html-xhtml-author-guide/html-xhtml-authoring-guide.html for an early draft. It's certainly possible to do, but it does require a good deal of coding discipline, and you will need to decide whether it's worth the effort. Although I create HTML4.01/XHTML1.0 polyglot documents, I create them using an XML tool chain which guarantees XML well-formedness and have specialized code to ensure compatibility with HTML non-void elements and valid XML characters. Direct hand coding would be very difficult.
One known current issue in HTML5 is the srcdoc attribute on the iframe element. Because the value of the attribute contains markup, certain characters need to be escaped. The HTML5 draft spec describes how to do this for the HTML serialization, but not (the last time I looked) how to do it in the XHTML serialization.
I'm late to the party but after 5 years the question is still relevant.
On one hand closing all my tags strongly appeals to me. For people reading it, for easier editing, for Great Justice. OTOH, looking at the gory details of the polyglot spec — http://www.sitepoint.com/have-you-considered-polyglot-markup/ has a convenient summary at the end — it's clear to me I can't get it all right by hand.
https://developer.mozilla.org/en/docs/Writing_JavaScript_for_XHTML also sheds interesting light on why XHTML failed: the very choice to use XML mime type has various side effects at run time. By now it should be routine for good JS code to handle these (e.g. always lowercase tag names before comparing) but I don't want all that. There are enough cross-browser issues to test for as-is, thank you.
So I think there is a useful middle way:
For now serve only as text/html. Stop worrying that it will actually parse as exactly the same DOM with same runtime behavior in both HTML and XML modes.
Only strive that it parses as some well-formed XML. It helps readers, it helps editors, it lets me use XML parser on my own documents.
Unfortunately, polyglot tools are rare to non-existant — it's hard to even serialize back XML in a way that also passes the HTML requirements...
No brainer: always self close void tags (<hr/>) and separately close non-void tags (<script ...></script>).
No brainers: use lowercase tags and attr (except some SVG but foreign content uses XML rules anyway), always quote attribute values, always provide attribute values (selected="selected" is more verbose than stanalone selected but I can live with that).
Inline <script> and <style> are most annoying. I can't use & or < inside without breaking XML parsing. I need:
<script>/*<![CDATA[*/
foo < bar && bar < baz;
/*]]>*/</script>
...and that's about it! Not caring about XML namespaces or matching HTML's implied DOM for tables drops about half the rules :-)
Await some future when I can directly go to authoring XHTML, skipping polyglotness. The benefits are I'll be able to forget the tag-closing limitations, will be able to directly consume and produce it with XML tools. Sure, neglecting xml namespaces and other things now will make the switch harder, but I think I'll create more new documents in this future than convert existing ones.
Actually I'm not entirely sure what's stopping me from living in that future right now. Is it only IE 8? I'm also a tiny bit concerned about the all-or-nothing error handling. I'm slighly hoping a future HTML spec will find a way to shrink the HTML vs XML gaps, e.g. make browsers accept <hr></hr> and <script .../> in HTML— while still retaining HTML error handling.
Also, tools. Having libraries in many languages that can serialize to polyglot markup would make it feasible for programs to generate it. Having tools to validate and convert HTML5 <-> polyglot <-> XHTML5 would help. Otherwise, it's pretty much doomed.
Given that the W3C's documentation on the differences between HTML and XHTML isn't even finished, it's probably not worth your time to try to do polyglot. Not yet anyways.... give it another couple of years.
In any event, only in the extremely narrow circumstances where you are actively planning on parsing your HTML as XML for some specific purpose, should you invest the extra time in XML-compliance. There are no benefits of doing it purely for consumption by web browsers -- only drawbacks.
Should you? Yes. But first some clarification on a couple points.
Sending the Content-Type: application/xhtml+xml header only means it should go through an XML parser, it still has all the benefits of HTML5 as far as I can tell.
About , that isn't defined in XML, the only character entity references XML defines are lt, gt, apos, quot, and amp, you will need to use numeric character references for anything else. The code for nbsp is or , I personally prefer hex because unicode code points are represented that way (U+00A0).
Sending the header is useful for testing because you can quickly find problems with your markup such as unclosed tags, stray end tags, text that could be interpreted as a tag, etc, basically stuff that can break the look or even functionality of your site.
Most significantly in my opinion, is if you are allowing user input and it fails to parse, that generally means you didn't escape their data and are leaving yourself open to a vulnerability. Parsed as HTML, you might not ever notice a problem until someone starts injecting scripts to harass your users or steal data.
This page is pretty good about explaining what polyglot markup is: https://blog.whatwg.org/xhtml5-in-a-nutshell
This sounds like a very difficult thing to do. One of the downfalls of XHTML was that it wasn't possible to steer successfully between the competing demands of XML and vintage HTML.
I think if you write HTML5 and validate it successfully, you will have as tidy and valid a document as anyone would need.
This wiki has some information not present in the W3C document: http://wiki.whatwg.org/wiki/HTML_vs._XHTML
i've got one more question today
are there any html parsers with not strict syntax analyzers available?
as far as i can see such analyzers are built in web browsers
i mean it should be very nice to get a parser that indulgently process the input document allowing any of the following situations that are invalid in xhtml and xml:
not self-closed single tags. for example: <br> or <hr>...
mismatched casing pairs: <td>...</TD>
attributes with no quotes marks: <span class=hilite>...</SPAN>
so on and so on... etc
suggest any suitable parser, please
thank you
TagSoup is available for various languages, including Java, C++ (Taggle) and XSLT (TSaxon).
...TagSoup, a SAX-compliant parser written in Java that, instead of parsing well-formed or valid XML, parses HTML as it is found in the wild: poor, nasty and brutish, though quite often far from short. TagSoup is designed for people who have to process this stuff using some semblance of a rational application design. By providing a SAX interface, it allows standard XML tools to be applied to even the worst HTML. TagSoup also includes a command-line processor that reads HTML files and can generate either clean HTML or well-formed XML that is a close approximation to XHTML.
If you're happy with Python, Beautiful Soup is just such a parser.
"You didn't write that awful page. You're just trying to get some data out of it. Right now, you don't really care what HTML is supposed to look like. Neither does this parser."
Hpricot is particularly good at parsing broken markup if you're not afraid of a bit of Ruby. http://github.com/whymirror/hpricot