This document serves a rationale document for various parts of the HTML5 specification. Over time this page will be a complete rationale document.
One Vendor, One Veto
Part of the the goal of the WHATWG is to document how web browsers actually handle HTML. As such browser vendors already have veto power - by not following the standard. The W3C and WHATWG do not have any enforcement power and can only write what browsers are willing to implement. Not removing features from the HTML standard that at least one browser vendor has stated they are unwilling to implement causes the HTML spec to not accurately document reality.
the <plaintext> element is a obsolete precursor to the <pre> element.  It is is now in the HTML5 spec as a method of stopping all further html token parsing. It lacks an end tag and just emits the rest of the page as plain text. It throws a parse error upon reaching the end of the document as it is not considered a valid element (and it is missing an end-tag).
<image> element is treated as an alternate (but invalid) name for <img>. This is because some sites (around 0.2%) make this mistake. It is already treated as an image by most major browsers.
Meter and Progress (are not the same thing)
<meter> is not just a special case of <progress>. The meter element represents a scalar measurement within a known range, such as storage quota usage, a relative popularity rating or relevance indicator. The control allows for the indication of high and low ranges, or minimum, maximum and optimal levels.
The progress element, on the other hand, represents the completion progress of a task. This could be a real time indicator for background processing task (e.g. using Web Workers or a file upload). Progress elmements can also be in the indeterminate state, indicating that something is in progress, but it's completion progress is unknown.
The default rendering for a meter element could look something like the following:
Whereas, the default rendering for the progress element could look like this:
Alternatively, an indeterminate progress bar could also be styled as a throbber, which indicates progress without any indication of the remaining progress:
See Re: <progress> draft for details.
B, I, EM, and STRONG
<em> is meant to indicate that some text is emphasized. <strong> is meant to confer importance upon text. <b> is meant for text that is stylistically offset from the rest of the text. Finally <i> is used to indicate that some text is meant to be read in an alternate mood.
Cats are <em>cute</em> animals.
could mean that cats are specifically cute.
Cats are <strong>cute</strong> animals.
could mean that the word cute is in some way important
Cats are <b>cute</b> animals.
could mean that the word cute is a new word (perhaps in a language lesson) but is not important
Cats are <i>cute</i> animals.
could mean that the word cute is meant to be read in a different tone (sarcastically for example)
IMG tag & alt text
The HTML parser has the following behavior difference in quirks mode:
- A start tag whose tag name is "table"
- If the Document is not set to quirks mode, and the stack of open elements has a p element in scope, then act as if an end tag with the tag name "p" had been seen.
- Why no namespaces
- Why no script implements
- Why not reuse legend or another mini-header element.
- Rationale for a variety of elements
- XHTML2 versus HTML5
- <meta http-equiv=content-language>
- http://lists.w3.org/Archives/Public/public-html/2009Jul/0257.html -- Re: Codecs for <video> and <audio></a>
- http://lists.w3.org/Archives/Public/www-archive/2009Jul/0075.html --Formal Objection to One vendor, One Veto
- Email from Ian Hickson; comment in spec source