4. Missing type Attribute in style Elements
Similarly, the type attribute must be included along with the style element when using embedded style in conforming documents. So, if you're doing this:
<style> h1 { color: red; } </style>
Stop doing that and make sure that you add the type attribute along with a value of text/css:
<style type="text/css"> h1 { color: red; } </style>
Of course, in an ideal situation, style information should be placed externally and linked via the link element. However, it's perfectly valid to use embedded style. Just make sure that you have the type attribute there and you're good to go.