Html5 specifies the <details> Element. It isn't widely supported and it doesn't look that great, but it is common to have functionality like the bootstrap collapse on a site.

I suggest an amp element that'll work similar.

<amp-details>
    <summary>Click here to see more.</summary>
    <p>This Text isn't initially visible.</p>
</amp-details>

This element may simply toggle an css class and let animations and other effects be handled by the author stylesheet (with a simple default stylesheet for display: none and display: block maybe.)