Learn · email signature glossary
TL;DR
Inline CSS means writing style rules inside each element's style attribute, like <td style="font-size:14px; color:#333333">, instead of defining them once in a <style> block or an external stylesheet. On the web this is considered poor practice because it repeats rules everywhere. In email it is mandatory: Gmail historically stripped <style> blocks entirely, and many clients, webmail front ends, and forwarding chains still remove or rewrite them. Styles attached directly to each tag are the only ones guaranteed to survive.
Re: In practice
A signature styled through classes and a stylesheet can look perfect in the editor where you built it and then arrive in the recipient's inbox as unstyled black Times New Roman, because the client discarded the stylesheet on the way in. Inlining removes that failure mode: each cell and link carries its own font, size, and color, so nothing depends on a document head that may not survive. The cost is verbose, repetitive markup, which is why email tooling typically lets you write normal CSS and then runs an inliner over it before sending. When you copy a signature between clients, inline styles are also what preserves formatting through the clipboard. If a signature loses its styling somewhere, non-inlined CSS is the first suspect.
Fwd: Worth a look
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