How to quote a specification like HTML5 in BibTeX

Here's a sample bibtex entry for a w3c page:

@TechReport{Maler:04:EML,
  author      = "Eve Maler and Jean Paoli and C. M. Sperberg-McQueen and Fran\c{c}ois Yergeau and Tim Bray",
  title       = "Extensible Markup Language ({XML}) 1.0 (Third Edition)",
  month       = feb,
  note        = "http://www.w3.org/TR/2004/REC-xml-20040204",
  year        = "2004",
  type        = "first Edition of a Recommendation",
  institution = "W3C",
}

The identification number for this page the 20040204, which you can see in the "note". The page link that you have given is currently under construction. Alternatively, you can select one of the versions and cite that.

This website generates BibTeX references for W3C documents. For example, given the URL

https://www.w3.org/TR/html53/

it currently produces:

@TechReport{Eden:18:H,
  author      = "Terence Eden and Xiaoqian Wu and Sangwhan Moon and Shwetank Dixit and Scott O'Hara and Patricia Aas and Bruce Lawson",
  title       = "{HTML} 5.3",
  month       = aug,
  note        = "https://www.w3.org/TR/2018/WD-html53-20180809/",
  year        = "2018",
  bibsource   = "http://w2.syronex.com/jmr/w3c-biblio",
  type        = "{W3C} Working Draft",
  institution = "W3C",
}

Note that it automatically resolves the URL (which points to the latest version) to the current version (20180809), so if you repeat this in the future you might get a different result. Also, this only works for documents published on www.w3.org; the dev.w3.org link in your question now redirects to w3c.github.io, which isn’t directly supported (you have to use the “latest published version” link from that document).