adding these two cases made the actual items jump around in font-size, on account of most items being the same text length and the audit using an unstable sort on that property. I added filler text to maintain the order.

just a style issue, but interleaving the offset makes this read a little more confusingly than it is. What about something like

let line = range.startLine + 1;
let column = range.startColumn;

// Add the startLine/startColumn of the <style> element to the range, if stylesheet is inline
// and a sourceURL magic comment is not present (`hasSourceURL` is true).
const addHtmlLocationOffset = stylesheet.isInline && !stylesheet.hasSourceURL;
if (addHtmlLocationOffset) {
  line += stylesheet.startLine;
  column += stylesheet.startColumn;
}

source += `:${line}:${column}`;

also may be worth calling out why sourceURL means you don't want the offset because it isn't obvious from this context, at least (admittedly I've paged all font-size stuff out of my brain :)

done and done

sourceURL's appear at the end of the block, by convention.. but they can show up at the top.

i think your code handles this situation, but FYI in case you want the coverage.

should be good, from what i know of the css parser. would just be testing the css parser (which I'm already doing perhaps too much here w/ the offset stuff ...)

i'd separate these concerns a little more but its no big deal really. calling out the +1 for lines but not columns is potentially worthwhile.

          let line = range.startLine;
          line++; // lines are 0-indexed in the protocol, but users expect 1-indexed line values;
          if (addHtmlLocationOffset) line += stylesheet.startLine;

ah nvm brendan's is nicer.