Hi

I'm still new to development and having a go with Twitter Bootstrap for the first time. Struggling to get my head around some of it, but my (possibly dumb) question is this:

I'm a bit confused by how non-semantic the framework is...at least in the tutorials given on the bootstrap site. How do html5 elements fit into this?

For example, is it ok to turn this....

<div class="row">
<div class="span12">
<h1>Content</h1>
</div>
</div>

...into this...


<div class="row">
<header="span12">
<h1>Content</h1>
</header>
</div>

??? (sorry I cant get the markup to indent on this post!)

And is it bad practice to modify the existing 'bootstrap.css' stylesheet, or should I customize the styles using a separate stylesheet?

Thanks