HTML formatted email not showing up at all in Gmail but is in other mail clients

I'm currently sending html formatted emails from PHP's mail() function and checking them simply to make sure they are showing up as HTML (formatting aside, I'm well aware that the formatting will vary (probably drastically) from client to client). They are displaying as HTML in the Apple Mail client as well as on my iPhone. However, Gmail simply refuses to display it as HTML as all. And by that I don't mean it is formatted incorrectly, it just simply isn't using HTML it appears.

The weird part is that an img from a tag is being loaded, but that's about it... is it that I'm mostly using divs for layout and that's too new for Gmail or something (vs tables)?

As far as CSS and what not goes, viewing the raw source shows the following:

<html><head><style>body { background-color: #F2F2F2; font-family: Arial; color: #5C5C5C; font-size: 11pt; } a { color: #46AAFF; text-decoration: none; } #container { width: 750px; margin-right: auto; margin-left: auto; } #header { float: right; padding: 3px; height: 30px; } #header a { color: #666666; margin-right: 10px; } #mainBody { padding: 20px; background-color: #FFF; border: 1px solid #DADADA; border-radius: 3px; } #imageArea { border-top: solid #E8E8E8 1px; margin: 15px; } #singleImageArea { width: 250px; padding: 15px; } #singleImageArea p { float: right; line-height: 30px; width: 130px; margin-top: 20px; } #footer { padding: 10px; color: gray; } #footer p { text-shadow: 0px 2px #FFF; font-size: 10pt; } #footer a { color: #ADADAD; float: right; padding-left: 20px; padding-right: 20px; } #footer img { width: 35px; height: 23px; padding: 10px; float: left; } </style></head><body><div id="container"><div id="header"><a href="http://appname.local/login/">Login</a></d
 iv><div style="clear: both;"></div><div id="mainBody"><h1>Hi Josh Holat!</h1>You've sent a request to <strong>reset</strong> your appname password. Just click the link below <em>(or copy and paste it into your browser)</em> and follow the instructions to set a new password. The link will expire in one hour.<br/><br/><a href="http://appname.local/sb/reset-password/?r=2768af61698fcde9c04f9449351575d6bfe6d720">http://appname.local/sb/reset-password/?r=2768af61698fcde9c04f9449351575d6bfe6d720</a><br/><br/>If you feel this request was submitted in error, don't worry; you can safely ignore this e-mail and your password will not be changed.<br/><br/>Much Love,<br/>Us</div><!-- #mainBody --><div id="footer"><img src="http://appname.local/images/email/footer_sb.png" /><p>&copy; 2011 appname, Inc. All Rights Reserved <a href="http://appname.local/sb/legal/">Legal</a><a href="http://appname.local/sb/contact/">Contact</a><a href="http://s
 tagebloc.local/sb/developers/">Developers</a><a href="http://appname.local/sb/blog/">Blog</a></p></div><!-- #footer --></div><!-- #container --></body></html>

As well as:

    Delivered-To: [email protected]
Received: by 10.229.40.2 with SMTP id i2cs133298qce;
        Sun, 28 Aug 2011 14:29:08 -0700 (PDT)
Received: by 10.42.152.199 with SMTP id j7mr4419937icw.417.1314566947950;
        Sun, 28 Aug 2011 14:29:07 -0700 (PDT)
Return-Path: <[email protected]>
Received: from joshholat.local (arh2281.urh.uiuc.edu [130.126.70.193])
        by mx.google.com with ESMTP id w3si6800550icz.109.2011.08.28.14.29.06;
        Sun, 28 Aug 2011 14:29:06 -0700 (PDT)
Received-SPF: neutral (google.com: 130.126.70.193 is neither permitted nor denied by best guess record for domain of [email protected]) client-ip=130.126.70.193;
Authentication-Results: mx.google.com; spf=neutral (google.com: 130.126.70.193 is neither permitted nor denied by best guess record for domain of [email protected]) [email protected]
Received: by joshholat.local (Postfix, from userid 70)
    id 775063662E01; Sun, 28 Aug 2011 16:29:06 -0500 (CDT)
To: Josh Holat <[email protected]>
Subject: Password Reset Request
X-PHP-Originating-Script: 501:Email.php
From: appname <[email protected]>
Content-type: text/html
Message-Id: <[email protected]>
Date: Sun, 28 Aug 2011 16:29:06 -0500 (CDT)

I'm confused as to why third party clients would show it fine but Gmail simply ignore it?

Solution

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/