standard Wagn pages don't validate as proper HTML, in part because they contain custom attributes (eg card-id here: <div card-id="123">).
use attributes that begin with the prefix "data-". this validates.
I started down this road with slot attributes (now in master). this probably wouldn't be a huge challenge.
note: this introduced a rule-editing bug. patched / deploying.
HTML on new wagns is completely valid.