* Support the <slot> tag
<slot> was added to the WHATWG HTML5 Living Standard on April 20, 2016.
* Also support the slot= attribute
Given that the <slot> tag by itself is not too useful, this commit also
introduces support for the global slot attribute.
* Using ParseBlock for <slot>
This is still suboptimal since ParseBlock will make it so that <slot>
always expects "Flow content", whereas the spec says that it should
have a Transparent content model.
In practice, it should allow all the cases that the spec allows for, but
it will also allow some cases that the spec does not allow. Notably, if
a <slot> tag is found in a Phrasing content (an inline context in
libtidy lingo), it will wrongly let Flow content (block tags in libtidy
lingo), whereas it shouldn't. But all in all, it's a good compromise.
This is a MUCH SANER approach to what I was trying to do (now that I screwed up enough internals to understand some of them!
At this point there are zero exit state reversions, and zero markup reversions! There are still 21 errout reversions; I'll
annotate and adjust as necessary.
the anchor name as a parameter, so it can look in the correct bin.
In the case of FreeAttrs, we have the name already (since we found a name or
id attribute). In the case of FixAnchors, the anchor name could come from
either the name or id attribute, so we call the function separately for each
case, passing the appropriate attribute value.
