Created attachment 425866 [details]
HTML example (alternative to CodePen)
## Steps to reproduce
1. Open https://codepen.io/36degrees/pen/LYxQBbm or the attached example.html
2. Hover over both links of the links on the line “17px link / with colour change” - text decoration thickness property expands to 3px
3. Hover over both of the links on the line “19px link / with colour change” - text decoration thickness property only expands to 3px on the link with colour change.
## Expected Behaviour
The text underline thickness should increase to 3px on all links when hovered.
## Actual Behaviour
In this example, the text-decoration-thickness only increases to 3px on hover on links up to a font-size of 19px, unless the link also has a colour change on hover.
The font-size at which the text-decoration-thickness property stops working changes depending on the thickness being set on hover. In the CodePen / attached example, text-decoration-thickness is set to 3px on hover and stops working at links of font-size: 19px. Other examples:
text-decoration-thickness on hover: 2px
font-size value at which it stops working: 15px
text-decoration-thickness on hover: 5px
font-size value at which it stops working: 27px
In Safari 14.1, the underlines for 19px+ links are not painted at all (but the color ones are). Not sure if we'd want to track this aspect as a separate bug.