* 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.
e51cd17c2d