Core CSS Issue

tl;dr
It appears that wordpress core is not generating some semicolons in the css which can cause rendering issues.

When reviewing a few websites I run, I noticed that some elements will randomly getting wrapped in a <b> element. I initially thought this was an issue with the theme builder I was using and posted on their forum. They advised me that this happens when there’s an opening tag missing a closing tag somewhere and provided https://validator.w3.org/ to check. Looking at all of my sites, I saw the following:

The line and column number may differ, but the issue was the same. Looking into the css further, I saw that there were a number of properties that were missing a semicolon, always the last property of an element:

As you can see, this issue appears on the wordpress.org site as well. I checked a number of other wordpress sites around the internet and found the same issue. The way I know this is an issue is because this isn’t the case with all css files on the same site:

I’ve tested this on multiple fresh installs, local and cloud hosting. No plugins, default theme.