Update 2016/10

Gmail updated the Gmail App so this may not apply to the latest version. We didn't check.

android-gmail-emulator

An hack to simulate the rendering by the Gmail App for Android in a simple browser

This is very partial, but contains the logic for which the Android app tries to make your email narrow by applying special "munged" classes. This does also try to sanitize the HTML the same way the APP does, but this DOES NOT clean the STYLE attribute at all (while there is a lot the Gmail app does in this regard).

A list of common smartphone viewport sizes