An HTML email can be generated correctly, accepted by a provider, and delivered successfully—then arrive with its most important image missing or transformed.
That is not only a design problem. It is an architectural reminder: email is a delivered document, not a miniature web page.
A web page runs in a renderer you can observe and update. An email leaves your boundary and enters clients that may block remote images, proxy them, cache them, rewrite markup, or alter colours for dark mode. When a visual matters to recognition or comprehension, a normal web asset reference can be a surprisingly weak contract.
The hidden web-page assumption
A remote image looks attractive because it keeps the message small and lets many messages share one cached asset. It also assumes several things outside your control:
