Solved Text underline

Hey everybody,
I'm trying to build an underline for a headline that reaches to the left edge of the browser window. To achieve this I'm using a row element with defined size in percent and absolute position which worked ok so far.
However as soon as the width of the browser window changes the line length (row width) changes not in relation to the headline length.
I know that this is due to the fact that the content area is limited to 80% with a max width of 1080px and center alignment. Since my underline is aligned to the left with a width of a certain percentage it automatically behaves differently to the headline.
Is there a way to kinda "align" the underline element to the headline so that it moves with it no matter the browser width?

I've also tried a different approach with a css box shadow but couldn't figure out how to get the underline to expand to the left browser edge.

I hope my question was clear enough (I'm still a css noob, sorry for that!)
Thanks a lot in advance!

 

Thanks for your reply! I decided to do it a different way in the meantime so the topic is kind of solved for me.