Backwards compatibility and DOCTYPE
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Backwards compatibility and DOCTYPE
by Bert Bos
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The HTML5 WD states (section 1.1.1[1]) that the format is meant to be as much backwards-compatible as possible. With a little change to section 8.1.1[2], HTML5 could, in fact, be fully backwards compatible. The current version (4.01) of HTML requires[3] documents to start with this DOCTYPE line: <!doctype html public "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> But that line is not allowed in the latest draft of version 5. Why not? The corresponding DOCTYPE lines from earlier versions of HTML can also be allowed. I think most documents that conformed to HTML when those DOCTYPE lines were current are still valid in HTML5 (once HTML5 allows those DOCTYPEs.) [1] http://www.w3.org/TR/2008/WD-html5-20080122/#relationship [2] http://www.w3.org/TR/2008/WD-html5-20080122/#the-doctype [3] http://www.w3.org/TR/1999/REC-html401-19991224/struct/global.html#h-7.2 Bert -- Bert Bos ( W 3 C ) http://www.w3.org/ http://www.w3.org/people/bos W3C/ERCIM bert@... 2004 Rt des Lucioles / BP 93 +33 (0)4 92 38 76 92 06902 Sophia Antipolis Cedex, France |
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Re: Backwards compatibility and DOCTYPE
by Frank Ellermann
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Bert Bos wrote: > The HTML5 WD states (section 1.1.1[1]) that the format is meant > to be as much backwards-compatible as possible. Among others <tt>, <s>, and <big> will be removed. The statement is marketing, e.g., for backwards-compatibility <s> is required. Various attributes including align= and clear= are also required for "Wilbur-browsers" and allowed in HTML 4, but not in HTML 5. > <!doctype html public "-//W3C//DTD HTML 4.01//EN" > "http://www.w3.org/TR/html4/strict.dtd"> > But that line is not allowed in the latest draft of version 5. > Why not? Because HTML 5 isn't HTML 4. With strict you are anyway not more compatible with legacy browsers. What problem are you trying to solve ? HTML 4 strict already removed <s>, align=, clear=, etc. Adding <tt>, <big>, etc. is no big deal if authors are determined to create pages for "popular browsers" instead of "any browser". HTML 5 apparently takes the approach to parse € up to Ÿ (or € up to Ÿ) as Windows-1252 0x80 up to 0x9F. HTML 5 parsers apparently do not insist on a <title>. HTML 5 is a kind of "GiGo", clearly defining what its garbage output will be for almost any garbage input found in the wild. 8.1.1 claims that the "doctype" is useless, and parses anything as "garbage input". "HTML 5" is not HTML, SGML, XML, or XHTML, what should it do with a real doctype, i.e. supposed to mean something, above ignoring it as in 8.1.1 ? Frank |
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Re: Backwards compatibility and DOCTYPE
by Ian Hickson
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On Wed, 5 Mar 2008, Bert Bos wrote: > > The HTML5 WD states (section 1.1.1[1]) that the format is meant to be as > much backwards-compatible as possible. With a little change to section > 8.1.1[2], HTML5 could, in fact, be fully backwards compatible. What do you mean by backwards compatible in this context? HTML5 doesn't claim that all legacy documents are conforming HTML5 documents (in fact no legacy documents are conforming HTML5 documents); it only claims that HTML5 user agents will process legacy documents in a manner compatible with legacy user agents. > The current version (4.01) of HTML requires[3] documents to start with > this DOCTYPE line: > > <!doctype html public "-//W3C//DTD HTML 4.01//EN" > "http://www.w3.org/TR/html4/strict.dtd"> > > But that line is not allowed in the latest draft of version 5. Why not? Because that line is HTML 4.01, not HTML5. If you want to write HTML 4.01, the HTML5 spec is not relevant. HTML5's UA requirements are compatible with that DOCTYPE, though, so a user agent written to HTML5 will process that document in a manner compatible with legacy user agents. -- Ian Hickson U+1047E )\._.,--....,'``. fL http://ln.hixie.ch/ U+263A /, _.. \ _\ ;`._ ,. Things that are impossible just take longer. `._.-(,_..'--(,_..'`-.;.' |
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Re: Backwards compatibility and DOCTYPE
by Bert Bos
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On Wednesday 05 March 2008 05:28, Ian Hickson wrote: > On Wed, 5 Mar 2008, Bert Bos wrote: > > The HTML5 WD states (section 1.1.1[1]) that the format is meant to > > be as much backwards-compatible as possible. With a little change > > to section 8.1.1[2], HTML5 could, in fact, be fully backwards > > compatible. > > What do you mean by backwards compatible in this context? HTML5 > doesn't claim that all legacy documents are conforming HTML5 > documents (in fact no legacy documents are conforming HTML5 > documents); Yes, and I wonder why. HTML5 can easily say that (most? all?) valid HTML4 document are also valid HTML5 documents. > it only claims that HTML5 user agents will process legacy > documents in a manner compatible with legacy user agents. That's not clear. The HTML5 draft says an incorrect DOCTYPE is an error and while some UAs may silently ignore the error, others may (or even must) report it. > > > The current version (4.01) of HTML requires[3] documents to start > > with this DOCTYPE line: > > > > <!doctype html public "-//W3C//DTD HTML 4.01//EN" > > "http://www.w3.org/TR/html4/strict.dtd"> > > > > But that line is not allowed in the latest draft of version 5. Why > > not? > > Because that line is HTML 4.01, not HTML5. If you want to write HTML > 4.01, the HTML5 spec is not relevant. Yes, it is. Once HTML5 is a REC, *it* defines HTML (see, e.g., sections 1.3 and 1.4.1) and HTML 4.01 is no longer relevant. It would be a pity if old documents suddenly stopped being HTML, when they only differ in a line that is "mostly useless" (as the draft says). It's nice that HTML5 takes forward compatibility into account (by not including a version number in document instances), but I don't see why it has to break with the past. I know previous versions of HTML had the same problem, but that is not a reason to repeat the mistake. > > HTML5's UA requirements are compatible with that DOCTYPE, though, so > a user agent written to HTML5 will process that document in a manner > compatible with legacy user agents. Bert -- Bert Bos ( W 3 C ) http://www.w3.org/ http://www.w3.org/people/bos W3C/ERCIM bert@... 2004 Rt des Lucioles / BP 93 +33 (0)4 92 38 76 92 06902 Sophia Antipolis Cedex, France |
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Re: Backwards compatibility and DOCTYPE
by Henri Sivonen
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Disclaimer: Not a WG response. On Mar 5, 2008, at 16:33, Bert Bos wrote: > On Wednesday 05 March 2008 05:28, Ian Hickson wrote: >> On Wed, 5 Mar 2008, Bert Bos wrote: >>> The HTML5 WD states (section 1.1.1[1]) that the format is meant to >>> be as much backwards-compatible as possible. With a little change >>> to section 8.1.1[2], HTML5 could, in fact, be fully backwards >>> compatible. >> >> What do you mean by backwards compatible in this context? HTML5 >> doesn't claim that all legacy documents are conforming HTML5 >> documents (in fact no legacy documents are conforming HTML5 >> documents); > > Yes, and I wonder why. HTML5 can easily say that (most? all?) valid > HTML4 document are also valid HTML5 documents. > Yes, it is. Once HTML5 is a REC, *it* defines HTML (see, e.g., > sections > 1.3 and 1.4.1) and HTML 4.01 is no longer relevant. It would be a pity > if old documents suddenly stopped being HTML, when they only differ in > a line that is "mostly useless" (as the draft says). First, I agree that there's no *technical* reason why HTML 4.01 could not be rescinded and *some* HTML 4.01-looking doctypes be made conforming as HTML5. > It's nice that HTML5 takes forward compatibility into account (by not > including a version number in document instances), but I don't see why > it has to break with the past. I know previous versions of HTML had > the > same problem, but that is not a reason to repeat the mistake. I've argued previously that in order for the current HTML5 to HTML6 forward-compatibility story to be believable, the same story should apply to the HTML4 to HTML5 transition: http://lists.whatwg.org/pipermail/help-whatwg.org/2008-February/000126.html Here are some complicating points in no particular order: * Making doctypes that say "HTML 4.01" conforming as HTML5 will confuse some people who have not yet internalized the notion that Web formats shouldn't have versions with which the language grows and shrinks but only levels that grow the language monotonically. * We shouldn't make quirks mode doctypes conforming, which means that some doctypes that were conforming under HTML 4.01 have to be non- conforming. * We probably shouldn't make the almost standard mode doctypes conforming, either, as long as the distinction between almost standards and standards modes is maintained in Gecko/Opera/WebKit. However, the almost standards mode is the common case among pragmatic standards-aware developers today. Therefore, it seems to me that in order to reap a notable practical benefit from making some legacy doctypes conforming, the CSS WG would need to fix the CSS defaults to match the almost standards mode so that the need for distinguishing the almost standards mode and the standards mode would go away. http://lists.w3.org/Archives/Public/www-style/2008Jan/0598.html * Valid HTML 4.01 formally includes various weird SGML stuff (minimizations in particular) that have never been supported by mainstream browsers. We shouldn't make that stuff conforming. Hence, we can't categorically make valid HTML 4.01 documents valid HTML5 documents. * HTML 4.01 has some above-SGML-layer bits that are so bad and rarely used that we should take the opportunity to ban them: The way isindex is implemented in browsers is fundamentally incompatible with the generally understood relationship of markup and the DOM. basefont is obsolete in practice. This is another reason why we shouldn't categorically make valid HTML 4.01 documents valid HTML5 documents. * On a number of other points mainly pertaining to presentational or redundant attributes (in particular border=0, target=_blank, language=JavaScript and presentational attributes on tables that were allowed in 4.01 Strict) I think we indeed should make more HTML 4.01 usage patterns conforming even if the patterns are no-ops or would have more elegant CSS alternatives. http://lists.w3.org/Archives/Public/public-html/2008Jan/0305.html http://hsivonen.iki.fi/test/moz/happycog-portfolio-results.txt -- Henri Sivonen hsivonen@... http://hsivonen.iki.fi/ |
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Re: Backwards compatibility and DOCTYPE
by Ian Hickson
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On Wed, 5 Mar 2008, Bert Bos wrote:
> On Wednesday 05 March 2008 05:28, Ian Hickson wrote: > > On Wed, 5 Mar 2008, Bert Bos wrote: > > > The HTML5 WD states (section 1.1.1[1]) that the format is meant to > > > be as much backwards-compatible as possible. With a little change > > > to section 8.1.1[2], HTML5 could, in fact, be fully backwards > > > compatible. > > > > What do you mean by backwards compatible in this context? HTML5 > > doesn't claim that all legacy documents are conforming HTML5 documents > > (in fact no legacy documents are conforming HTML5 documents); > > Yes, and I wonder why. HTML5 can easily say that (most? all?) valid > HTML4 document are also valid HTML5 documents. by claiming that documents labeled "HTML4" were in fact HTML5. Going forward we've resolved this -- since HTML5 has no version number in the source, HTML5 and HTML6 can't be distinguished, and HTML6 can safely take over HTML5's documents and wipe HTML5 off the face of the earth. :-) > > it only claims that HTML5 user agents will process legacy documents in > > a manner compatible with legacy user agents. > > That's not clear. The HTML5 draft says an incorrect DOCTYPE is an error > and while some UAs may silently ignore the error, others may (or even > must) report it. Yeah, a better way of phrasing it is that the HTML5 draft says how HTML5 user agents can process legacy documens in a manner compatible with legacy user agents. (Error recovery is not required in all cases, e.g. when parsing, because some use cases -- such as preprocessors intended for developers who have control over the markup -- don't need to handle broken code, since the developer is right there to fix it.) > > > The current version (4.01) of HTML requires[3] documents to start > > > with this DOCTYPE line: > > > > > > <!doctype html public "-//W3C//DTD HTML 4.01//EN" > > > "http://www.w3.org/TR/html4/strict.dtd"> > > > > > > But that line is not allowed in the latest draft of version 5. Why > > > not? > > > > Because that line is HTML 4.01, not HTML5. If you want to write HTML > > 4.01, the HTML5 spec is not relevant. > > Yes, it is. Once HTML5 is a REC, *it* defines HTML (see, e.g., sections > 1.3 and 1.4.1) and HTML 4.01 is no longer relevant. It would be a pity > if old documents suddenly stopped being HTML, when they only differ in a > line that is "mostly useless" (as the draft says). are still conforming HTML4 docs. > It's nice that HTML5 takes forward compatibility into account (by not > including a version number in document instances), but I don't see why > it has to break with the past. I know previous versions of HTML had the > same problem, but that is not a reason to repeat the mistake. I think that HTML5 claiming that documents that say "HTML4" are actually HTML5 is something that would be much harder to sell. In practice, it doesn't really matter much. -- Ian Hickson U+1047E )\._.,--....,'``. fL http://ln.hixie.ch/ U+263A /, _.. \ _\ ;`._ ,. Things that are impossible just take longer. `._.-(,_..'--(,_..'`-.;.' |
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