Preserve whitespace in DOMDocument::loadHTML() calls

Problem/Motivation

libxml2 prior to 2.9.0 removed blank elements (whitespace) when parsing HTML. This behaviour was reverted in later versions of libxml2 in https://gitlab.gnome.org/GNOME/libxml2/-/commit/f933c898132f20a50ba39ac6...

In #3204929: Html::load() inconsistent space removal with old libxml2 versions. we added LIBXML_NOBLANKS to DOMDocument::loadHTML() calls to preserve compatibility with the previous behaviour, so our tests pass the same way on both the old and new versions.

We should decide whether stripping whitespace is the right thing to do, or whether we should now preserve whitespace and update our tests to match the new behaviour.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet