This behaviour is new in Safari 14 (14.1). Example issue: a <SPAN> element has font-size set to 16px and line-height set to 1.2 (via CSS). The resulting height of the element was correctly computed to 19.2 (prior v14) but is now resulting 19.
Created attachment 428381 [details] Safari 14.1 Inspector Size is reported as 19px
Created attachment 428382 [details] Firefox 88.0.1 Inspector
Created attachment 428383 [details] Test element
<rdar://problem/78201712>
Is this? https://wpt.fyi/results/css/CSS2/linebox/fractional-line-height.html?label=master&label=experimental&aligned=&q=fractional+line+height