We set a global `min-height: 100%` for the body element (a common strategy to
allow for background images to fill the viewport), but this is undesirable for
iframes where the `window.onresize` is used to programmatically adjust the
iframe height. Because `onresize` will fire when the body adjusts itself as
per its `min-height: 100%`, in certain situations this will lead to an infinite
resizing loop. This is notably visible when:
* Rendering the video block when using Firefox: this causes the iframe
size to jiggle indefinitely.
* Rendering the HTML block when there are empty block elements (such as
`<p></p>`) that have a vertical margin set (which happens to be the
default to HTML blocks): this will cause the iframe to increase in
height infinitely.
By adding an inline `min-height: auto` on the wrapping body element,
this overrides the Studio-served CSS, and thus fixes the problem.
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