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Re: Backwards compatibility and DOCTYPE

From: Ian Hickson <ian@hixie.ch>
Date: Wed, 5 Mar 2008 04:28:09 +0000 (UTC)
To: Bert Bos <bert@w3.org>
Cc: public-html-comments@w3.org
Message-ID: <Pine.LNX.4.62.0803050425270.6407@hixie.dreamhostps.com>

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.   `._.-(,_..'--(,_..'`-.;.'
Received on Wednesday, 5 March 2008 04:28:22 UTC

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