Hey 脕d谩m S.Aurelien S. Here's my Explanation & Preview Link to Copy the Code 馃殌Preview LinkWebflow Link
TLDR:
Add a span with class underline in heading, keep the span position relative, add the CSS ::after Property to add width & position variables.
Text Shadow is optional if you want to do that line going under the text thing. since you need to adjust that in responsive viewports!
Only "Amazing" Explanation I found is from Finsweet team, they streamed 2 year back.
Scrub to 9-10 min in video to see the actual implementation.
https://youtu.be/_Cc123WmZw4