Annotate / Highlight buttons stretched in Safari on surma.dev

**Steps to reproduce:**

1. Visit https://surma.dev/things/ditherpunk/ and activate Hypothesis (eg. with the bookmarklet, or by visiting the site with Via).
2. Select some text to trigger the adder

**Expected result:** Adder appears, looks normal.
**Actual result:** In Chrome and Firefox, it works as expected. In Safari 14, the buttons are wider than normal and the font is distorted:

Tested with Chrome 1.629.0.

This is unexpected because Safari 14 supports Shadow DOM, so the adder's styles should be isolated from the influence of the page.