- Remove `seamless` attribute which has not been supported by major
   browsers for years [1]
 - Remove the `name="hyp_sidebar_frame"` attribute which is not
   referenced anywhere else in the code. I did find a couple of
   references to it in Google, but only in scripts written by Hypothesis
   team members which are not run regularly.

[1] https://caniuse.com/iframe-seamless