invalid HTML output for <gap> within <rs> 

See e.g. A046277, where the input TEI results in nested html:a tags:

## TEI 

```xml
<rs type="person" key="A008673">Zepf<gap reason="illegible"/>chen</rs>
```

## HTML

```html
<a class="preview persons A008673" href="">Zepf<span>[…]<a class="noteMarker" id="ref-d10e117" data-toggle="popover" data-trigger="focus" tabindex="0" data-ref="#d10e117">‡</a></span>chen</a>
```