I have been testing responsive email in the Gmail app on Android and my code wasn't displaying responsive. Everytime I sent myself a test the email would display the desktop version and not the mobile responsive version. After playing around and tweeking I figured out that css has to be written with class selectors .header {width:320px !important;} and you can't use attribute selectors table[class="header"]{width:320px !important;}

I went through my code and changed all the classes and then when I retested it finally rendered responsive in the Gmail app on Android!

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