Which inline HTML styles does GitHub Markdown accept?

After GitHub converts Markdown to HTML,

The HTML is sanitized, aggressively removing things that could harm you and your kin—such as script tags, inline-styles, and class or id attributes. See the sanitization filter for the full whitelist.

You’re right: style tags are not included in the whitelist. They are ignored.

The permitted attributes differ by tag. I recommend referring to the sanitization filter link above. It leads to a fairly readable Ruby source file.