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 XHTML 1.0 was last revised in 2002 and was superseded in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]