Crosspost note. Originally published at min8t.com/articles/inline-css-for-email . I'm one of the maintainers of MiN8T, which makes the in-browser CSS inliner referenced at the end. The technical content here applies regardless of which inliner you reach for.

You write CSS in a <style> block, the way you have for two decades on the web. You hit send. Half your subscribers see your beautiful design. The other half see something else : a colorless wall of text, a stack of mis-aligned images, a call-to-action that has lost its rounded corners and bright background and is now indistinguishable from the body copy.

The reason: most email clients strip or partially strip <style> blocks. Gmail mobile, Yahoo, AOL, several enterprise clients. They discard your stylesheet entirely. The fix is older than the problem: inline every style as a style="..." attribute on the element it targets .…