Steps to reproduce:
1. Go to https://wpt.fyi/results/css/css-color?label=experimental&label=master&product=chrome&product=firefox&product=safari&aligned&view=interop&q=label%3Ainterop-2022-color
2. With web inspector opened, with the crosshair tool, select a row of the table (with the test results)
3. Put the cursor in Elements on a "tr" element
4. Click on the "+" at the bottom of the style rules column (near the Filter search field on the left)

Expected:
tr is added to the column rule

Actual:
Nothing. No error message (but + button is active)
The content is part of the shadow content and Web Inspector can't be used to add styles to it.