AMP is so fast is because it restricts how pages may be coded, and the AMP Validator will invalidate pages that do not meet the necessary requirements. It is important to realise that a customer’s site design may not translate perfectly into an AMP page, or deliver all the same functionality. Here are some areas where you’ll need to keep these limitations in mind:
No external CSS is permitted on the AMP document, and inline CSS cannot exceed 50kb. This can affect a design-rich site design that requires a lot of intricate styling and code-heavy SVGs.
The way AMP renders anything other than system fonts can put restrictions on the site design. AMP can render webfonts from external sources, however anymore more than a single font and weight may cause problems with HTML file size, causing the AMP validator to fail.
AMP restricts animations to two methods, transform and opacity. Most animations can be made to work using these two properties but designers should be aware if the design relies on more complex animation techniques to work.