I have a page working in all browsers including IE 8/9 except when those browsers shift to compatibility mode – then any text tagged as <p> becomes horribly oversized – larger than <h1>. The <p> is not responding to any stylesheets or css directives as pertains to size.
jquery and jQueryUI are present on the page but not involved in (explicitly) styling my <p> elements.
Jukka’s premise proved correct – an unclosed H1 tag was the cause.