James here from DocRaptor. This is a common issue our users are running into. Prince doesn't appear to properly support either of the industry standard "clearfix"s, which are used by Bootstrap and most other CSS frameworks.
A version of this code (normally applied in an :after psuedo-element) is the standard clearfix:
<div style="display: table; clear: both; line-height: 0;"></div>
That code should clear the float and make the parent div in the attached examples have a red background.
James Paden - DocRaptor
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Edited by jamespaden