HTML formatted email not showing up at all in Gmail but is in other mail clients
GMail doesn't support <style> tags and neither does a number of clients. To make your life easier, you can use convert to convert your style tags to inline CSS: https://templates.mailchimp.com/resources/inline-css/
The above answers are pretty good, just wanted to add a few more points. All credits to this link:
Gmail only supports
<style>in the<head>. Gmail does support embedded styles (<style>). However, embedded styles are only supported in the head of your HTML document. The Gmail Android and iOS apps does not support<style>at all when rendering emails retrieved through non-Google accounts (GANGA).Gmail removes your entire
<style>block if it encounters an error.Gmail does not allow negative CSS margin values.
Gmail does not support attribute selectors and most pseudo-classes. e.g.
div[class="content"]{ color: red }. Gmail also does not support pseudo-classes like:checkedand:active.
Edit: in retrospection after 3 years, Gmail CSS is pretty weird.
I see you're using an ID named #footer. It seems that Google doesn't recognize that name when dealing with mails. Notice that in the font code seen from gmail, all your ID might appear with "exc" + idName, except #footer. Try to use a different name.
I am updating my answer since it has quite a few views, and new tools are available. I am leaving my original answer for posterity.
Edit * 9/19/19
You can find CSS property support by email client here: https://www.caniemail.com/
Original Answer:
Gmail does not support the <style> tag. You need to use inline css for gmail to work correctly.
Here's a reference list. http://www.campaignmonitor.com/css/