Comment 2 for bug 1758553

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.