I've found that in the Chrome and Firefox mobile browsers, an h1 over a certain length increases the font size of all statically-positioned text on the page but not absolutely-positioned text. The length of the h1 required to trigger the problem seems to depend on the browser and (probably) the device.

For example, on a Nexus 4 using Chrome or Firefox, the h1 text and statically-positioned div text from the following code renders too large while the absolutely-positioned div text renders in normal size. Removing 2 characters of text from the h1 tag causes all text to render in normal size in Chrome. Removing 2 more characters causes all text to render in normal size in Firefox.

Code:
<html>
<body style="margin: 0;">
<h1 style="font-size: 1em;">h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1 h1</h1>
<div style="font-size: 1em;">div</div>
<div style="position: absolute; font-size: 1em;">absolute</div>
</body>
</html>

Does anyone know why this happens and how to control it?