The overflow hidden removed in 72f8fd971d did cut of iframes and their content. However, it did introduce a scrollbar when the iframe is too big. Which is fixed here by adding a max-width of 100%.
There was a max-width but one with fixed pixel values before, which was removed in 345283782c (diff-5470058378896897263b7a99e4226772660e09d5e9e51b530fffc6075b8e07bfL1299).
The stylesheet-style's max-width will overwrite any width-specification given via the width-attribute on the iframe as well as width-conditions set as inline-styles. However, adding a inline max-width-style will overwrite the css-file specification due to higher css specificity.
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