<source> and <track> are void elements in HTML 5

[Vincent Bernat]

Hey!

In HTML5, the void elements are listed here: https://www.w3.org/TR/html5/syntax.html#void-elements. This doesn't match the list in src/lxml/html/defs.py. Notably, <source> and <track> are in html5_tags instead.

[scoder (scoder)]

Implemented here:
https://github.com/lxml/lxml/commit/1e6007745376593cd10ca2389aa6bc406f72f630

[Vincent Bernat (vbernat)]

Hey!

Sorry for not testing earlier, but it doesn't seem to work:

```
>>> from lxml import etree as ET
>>> import lxml.html
>>> lxml.html.tostring(ET.Element("source"))
'<source></source>'
>>> lxml.html.tostring(ET.Element("img"))
'<img>'
```

Maybe these tags should be removed from `html5_tags`? All other tags from `empty_tags` are part of another set. Dunno, if this is mandatory.

[scoder (scoder)]

The serialisation is done by libxml2. defs.py is mostly used by the cleaner and for informational purposes, not for serialisation.

[Vincent Bernat (vbernat)]

Thanks, I will push a similar fix to libxml2 then!