element 'u' is not allowed for content model

```
FLightcrew Errors out on the tags within Sigil.

element 'u' is not allowed for content model

'(a|br|span|bdo|map|object|img|svg|tt|i|b|big|small|em|strong|dfn|code|q|samp|kb
d|var|cite|abbr|acronym|sub|sup|input|select|textarea|label|button|ins|del|scrip
t)'

It has all tags bar the underline ().

```

Original issue reported on code.google.com by `shaun.voysey@outlook.com` on 1 Mar 2012 at 7:41

Reproduce the validation error in Sigil using an empty element and trace the content-model check that rejects it. Done means the underline element is accepted alongside the other listed inline elements without producing this error.

Written by the indexing model from the issue text.