Outlook 365 web: what breaks in an HTML email signature

Almost no client loads web fonts. Only Arial, Helvetica, Georgia, Times New Roman, Verdana, Tahoma, Trebuchet MS and Courier New are safe.

Outlook for Windows renders with the Word engine, which knows neither flex nor grid. Columns stack and the signature falls apart.

When a signature is pasted into the Gmail settings editor, Gmail discards the entire <style> block. Every declaration has to live in a style attribute.

Dropped silently: you do not see an error, you see a gap. Anything that must be visible goes in an <img>.

Reliable spacing comes from padding inside the cell, or from spacer rows with a fixed height.

Without both attributes, Outlook reserves the wrong space and the signature jumps as it loads.

The only layout method that works in all seven. This is why serious email signatures are still built with tables in 2026.

The most expensive mistake in this category: Gmail refuses to render base64 images. A signature that looks perfect in Outlook arrives broken for half the world.

A round photo comes out square in Outlook desktop. It degrades without breaking, but you need to know before you promise it to a client.

Ignored in Outlook desktop. Degrades cleanly, so use it as reinforcement, never to make something fit.

Unreliable in Outlook desktop. Type the capitals into the text itself instead of applying them with CSS.

Outlook desktop ignores max-width. The outer table needs an absolute pixel width, 600 at most.