You've always been able to do it, doesn't even need to be HTML, serialise as XHT...

You've always been able to do it, doesn't even need to be HTML, serialise as XHTML and you can include the tags on your own namespace and have separate CSS for them.

<!DOCTYPE html>

<html

        xmlns="http://www.w3.org/1999/xhtml"

        xmlns:ns1="mynamespace"

        xmlns:ns2="yournamespace"

<body>

<CustomElement>Hello</CustomElement>

<ns1:CustomElement>Hello</ns1:CustomElement>

<ns2:CustomElement>Hello</ns2:CustomElement>

    @namespace ns1 "mynamespace";

    @namespace ns2 "yournamespace";

    CustomElement {

        color:green;

    }

    ns1|CustomElement {

        color:red;

    }

    ns2|CustomElement {

        color:blue;

    }

</style>

</body>

</html>