The `html5` option was added in 2010 and marked as "not experimental" in 2011
but is not the default.

According to <http://caniuse.com/#feat=html5semantic>, current versions of
all recent versions of all major browsers - even IE (9+) - support the HTML5
semantic elements (`<section>` etc.), except for `<main>` which IkiWiki
doesn't use anyway. However, IE 8 is not a current version, but has ~ 4%
market share and doesn't support `<section>` and friends; so there's still
a compatibility concern there.

In particular, non-HTML5 mode uses `<!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">`
which doesn't allow newer markup like the `role` attribute, so we can't close
[[todo/add_aria_landmarks_to_make_ikiwiki_websites_more_accessible]] while
remaining XHTML 1.0 Strict. The recommended pseudo-doctype for HTML5, and for
HTML with ARIA markup, is `<!DOCTYPE html>`.

(I do think we should continue to use `<xml-compatible-tags />` and output
well-formed XML so people who want to do XSLT tricks with IkiWiki's output
can do so, though.)

In practice, real browsers have never actually implemented a strict XHTML mode:
they've always parsed `text/html` as "tag soup", because they need a tag-soup
parser anyway, and nobody wants to maintain two parsers.

Kai also wants a HTML5 doctype for [[bugs/more mobile friendly default themes]].

Options include:

* set html5 to 1 by default but retain the dual-mode templates,
  breaking IE 8 by default

* remove the option and always behave as if it had been 1, simplifying
  the templates and breaking IE 8 unconditionally

* either of the above and include
  [html5shiv](https://code.google.com/p/html5shiv/) to de-break IE 8

* change the doctype to `<!DOCTYPE html>`
  unconditionally, stop trying to limit ourselves to XHTML 1.0 Strict
  (use HTML5 features that degrade gracefully, like
  [[ARIA roles|todo/add aria landmarks to make ikiwiki websites more accessible]]),
  but avoid using the new elements like `<section>` that require specific
  browser support unless `html5` is set to 1. That would get rid of the
  backwards-compatibility concerns while keeping the ability to use
  post-2000 markup; we can use `html5` to mean "be more enthusiastic about
  HTML5 features even if they might fail on older browsers".

Using the HTML5 doctype does mean we lose the ability to validate the output
against a DTD (as `wdg-html-validator` does), but DTDs have very little to
do with practical browser compatibility in any case.

[[!template id=gitbranch branch=smcv/ready/html5
author="[[Simon McVittie|smcv]]"
browse=http://git.pseudorandom.co.uk/smcv/ikiwiki.git/shortlog/refs/heads/ready/html5]]
[[!tag patch users/smcv/ready]]

At the moment my preferred option is the last, for which see my `ready/html5`
branch. I'll apply this at some point if there are no objections.

--[[smcv]]

> [[merged|done]] --[[smcv]]
