whether to allow deprecated elements (YES is faster here for Element nodes)
Description:
Checks whether the tree node is valid. Experimental (the author only uses the HTML enhancements in a SAX parser).
Return value:
For Element nodes, a return from htmlElementAllowedHere (if legacy allowed) or htmlElementStatusHere (otherwise). for Attribute nodes, a return from htmlAttrAllowed for other nodes, HTML_NA (no checks performed)