Invalid Elements can definitely be built (non-allowed child
elements), but still has public fields since it will be plucked
apart and verified in allocate_element before being stored. And
there’s no mut access to the store.
Serialized HTML fragment string. Can be included in
Node:s. Contains the metainformation about the outermost element
in the serialized fragment for dynamic DOM checking, and an
Arc<str> with the serialized data; SerHtmlFrag is thus
(reasonably) cheap to clone. This has pub fields for special
needs, be careful not to mis-use. The string needs to contain the
top-most tag in serialized form, too, and it should match the one
specified as meta.