Tw_html.AtSourceFunctions for creating HTML attributes.
spellcheck value sets the spellcheck attribute. Use "true" or "false".
if' cond at is at if cond is true, otherwise returns a void attribute.
to_pair at returns the attribute as a (name, value) pair.
of_pair (n,v) creates an attribute from a (name, value) pair.
hidden hides the element.