W3C and WHATWG have been characterized as both working together on the development of HTML 5, and yet also at cross purposes ever since the July 2012 split of the W3C work into milestone-based static standards and WHATWG's into a continually updated "living standard". The relationship has been described as "fragile", even a "rift", and characterized by "squabbling".
In at least one case, namely the permissible content of the
<cite></cite> element, the two specifications directly contradict each other (as of July 2018), with the W3C definition being permissive and reflecting traditional use of the element since its introduction, but WHATWG limiting it to a single defined type of content (the title of the work cited). This is actually at odds with WHATWG's stated goals of ensuring backward compatibility and not losing prior functionality.
The "Introduction" section in the WHATWG spec (edited by Ian "Hixie" Hickson) is critical of W3C, e.g "Note: Although we have asked them to stop doing so, the W3C also republishes some parts of this specification as separate documents." In its "History" subsection it portrays W3C as resistant to Hickson's and WHATWG's original HTML 5 plans, then jumping on the bandwagon belatedly (though Hickson was in control of the W3C HTML 5 spec, too). Regardless, it indicates a major philosophical divide between the organizations:
For a number of years, both groups then worked together. In 2011, however, the groups came to the conclusion that they had different goals: the W3C wanted to publish a "finished" version of "HTML5", while the WHATWG wanted to continue working on a Living Standard for HTML, continuously maintaining the specification rather than freezing it in a state with known problems, and adding new features as needed to evolve the platform.
Since then, the WHATWG has been working on this specification (amongst others), and the W3C has been copying fixes made by the WHATWG into their fork of the document (which also has other changes).
The "markets" for the two specifications are largely different. The W3C spec is the one that Web developers most often refer to, while the WHATWG version is used by the software development teams of the browser makers (though a version exists for Web content authors, trimmed of the material only of interest to browser coders). New features are added to HTML and, often experimentally, to browsers long before they appear in a W3C spec, because they arise in the WHATWG one. The technology journal Ars Technica observed that "both groups are likely to continue to exist, and both groups will continue to have broad-based industry backing".
In addition to the contradiction in the
<<a href="https://en.wikipedia.org/wiki/HTML_elements#cite" title="HTML elements">cite</a>> element mentioned above, other differences between the two standards include at least the following, as of September 2018:
Content or Features Unique to W3C or WHATWG Standard
|Site pagination||Single page version (allows global search of contents)|
§10 Web workers
§11 Web storage
|Global attributes||: ||: |
|Chapter Elements of HTML||§4.13 Custom elements|
 is in section Grouping content.
|§ ||§126.96.36.199. Other pragma directives, based on deprecated WHATWG procedure.|
|§ Sections||§ 188.8.131.52 Sample outlines|
§ 184.108.40.206 Exposing outlines to users
|Structured data||Recommends RDFa (code examples, separate specs, no special attributes).||Recommends Microdata (code examples, spec chapter, special attributes).|
The following table provides data from the Mozilla Development Network on compatibility with major browsers, as of September 2018, of HTML elements unique to one of the standards:
|W3C||All browsers, except Edge|
|W3C||None, except Firefox|
|WHATWG||All browsers||"[Since] the HTML outline algorithm is not implemented in any browsers ... the |
|WHATWG||Full support only in Edge and Firefox desktop.|
Partial support in Firefox mobile.
Supported in Opera with user opt-in.
Not supported in other browsers.
|WHATWG||All browsers, except Edge and IE||Experimental technology|