Email clients typically support only a subset of valid HTML and do not have strong support for CSS (especially CSS in the <head> of the HTML email). Yahoo, Outlook, and even Gmail strip the CSS included in the <head> of your HTML. In most cases, CSS for email is handled using inline styles.
Rather than inlining your CSS manually before sending your HTML to Transactional Email or when you generate a template, we offer a sending option to inline CSS automatically at the time of send (as long as your message is 256KB or smaller). Set this on the Sending Defaults page in your Transactional Email account, on a per-message basis using the inline_css option (API) or X-MC-InlineCSS header (SMTP), or using the Transactional Email Rules Engine.
CSS inlining can be an intensive process and because Transactional Email is designed to get your emails out as quickly as possible, the extra processing time to inline CSS can affect delivery speeds. If you have a larger template or HTML document, we recommend that you inline your styles manually or use the CSS Inliner Tool as a part of your sending process.
Transactional Email will inline CSS only for HTML emails smaller than 256KB.