Html5 audio and video tag

support for the HTML5 audio and video tag. Pull-request created to track progress.
This is wrong, `preload` is not a "Boolean attribute" (in the sense of 879cb91cb81043b02478fb2ff60001d81f30c0d7, e.g. attribute `x` only valid without value (`<tag …="…" x>`) or with value equal to the attribute name (`<tag …="…" x="x">`). `preload="preload"` is not even valid.