[PATCH v2] gitweb: switch to an XHTML5 DOCTYPE

According to the HTML Standard FAQ:

	“What is the DOCTYPE for modern HTML documents?

	In text/html documents:

		<!DOCTYPE html>

	In documents delivered with an XML media type: no DOCTYPE is required
	and its use is generally unnecessary. However, you may use one if you
	want (see the following question). Note that the above is well-formed
	XML.”

	Source: [1]

Gitweb uses an XHTML 1.0 DOCTYPE:

	<!DOCTYPE html PUBLIC
	"-//W3C//DTD XHTML 1.0 Strict//EN"
	"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

While that DOCTYPE is still valid [2], it has several disadvantages:

1. It’s misleading. If an XML parser uses the DTD at the given link,
   then the entities   and ⋅ won’t get declared. Instead, the
   parser has to use a DTD from the HTML Standard that has nothing to do
   with XHTML 1.0 [2].
2. It’s obsolete. XHTML 1.0 was last revised in 2002 and was superseded in
   2018 [3].
3. It’s unreliable. Gitweb uses   and ⋅ but lets an external file
   define them. “[…U]using entity references for characters in XML documents
   is unsafe if they are defined in an external file (except for <, >,
   &, ", and ').” [4]

[1]: <https://github.com/whatwg/html/blob/main/FAQ.md#what-is-the-doctype-for-modern-html-documents> [2]: <https://html.spec.whatwg.org/multipage/xhtml.html#parsing-xhtml-documents> [3]: <https://www.w3.org/TR/xhtml1/#xhtml> [4]: <https://html.spec.whatwg.org/multipage/xhtml.html#writing-xhtml-documents>

Signed-off-by: Jason Yundt <jason@jasonyundt.email>
---
Compared to the first version of this patch, this version:
1. makes it clear that XML parsers may used the linked DTD like brian
   mentioned.
2. mentions HTML5 like Bagas suggested.

 gitweb/gitweb.perl                        |  5 ++++-
 t/t9502-gitweb-standalone-parse-output.sh | 14 ++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)