Allow common safe HTML5 elements and attributes.
This list is based off of the Caja whitelists at: https://code.google.com/p/google-caja/wiki/CajaWhitelists.
Common things which are not allowed are script elements, style attributes and any script handlers.
Source
void allowHtml5({UriPolicy uriPolicy}) {
add(new _Html5NodeValidator(uriPolicy: uriPolicy));
}