Sometimes, warnings may appear on my stories saying that AMP is not valid:

Hovering over a warning with the mouse will display a tooltip, detailing the reason for the warning and linking to the relevant documentation.
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The most common reason is that CSS styles (pre-defined and inline combined) have gone over the allowed 50,000 byte limit. In this case, it may be necessary to reduce the number of scenes or fonts used, until the warning disappears.