Per https://www.w3.org/TR/html5/syntax.html#the-doctype the HTML5 doctype can be one of
Currently it seems only the first one is supported. Please consider adding support for the DOCTYPE legacy string.
<!DOCTYPE html>
<!DOCTYPE html SYSTEM "about:legacy-compat">
<!DOCTYPE html SYSTEM 'about:legacy-compat'>,
Currently it seems only the first one is supported. Please consider adding support for the DOCTYPE legacy string.