Addition to BeNdErR's answer:
The "other TEXT" element should have float:none
, like:
<div style="width:100%;">_x000D_
<div style="float:left;width:30%; background:red;">...something something something random text</div>_x000D_
<div style="float:none; background:yellow;"> text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text </div>_x000D_
</div>
_x000D_