html – Uncommon high CPU use in web browser set off by a design change

The trouble takes place in both Chrome and also Firefox, on a current Ubuntu 20.4 system.

I have a web page including just web links, and also just recently included this code to it:

<< design>>.

{a white-space: nowrap; border-bottom-color: var(-- c-highlight);}

<.

...

anchor.onmousedown = feature( b) {
if (b.button <