Font-weight:bolder for fonts with multiple weights

Improvement for fonts that have multiple weights.  
If some block, for example header, is bold and has `strong` inside we can make `strong` bolder if font has additional weights.
 
```html
<h1>Header with <strong>nested <strong>strong</strong> inside</strong></h1>
```

Check this [demo](http://cssdeck.com/labs/x20utix0).
Please improvize the comment as a fix for Safari