The archive.org embed renders content at native pixel dimensions and
doesn't scale to fill the iframe like YouTube does. On mobile the
550px-wide content overflowed the ~370px container.
Fix: give the iframe its native dimensions and use transform: scale()
with a ResizeObserver to shrink it to fit the container width. Also
reverts the unnecessary overflow-x-hidden on ExternalContentPage.
Alex Gleason
8b73772bf0