gecko-dev/layout/style/test/browser_sourcemap.js

add_task(async function() {
  let uri = "http://example.com/browser/layout/style/test/sourcemap_css.html";
  info(`URI is ${uri}`);

  await BrowserTestUtils.withNewTab(
    {
      gBrowser,
      url: uri,
    },
    async function(browser) {
      await SpecialPowers.spawn(browser, [], function() {
        let seenSheets = 0;

        for (let i = 0; i < content.document.styleSheets.length; ++i) {
          let sheet = content.document.styleSheets[i];

          info(`Checking ${sheet.href}`);
          if (/mapped\.css/.test(sheet.href)) {
            is(
              sheet.sourceMapURL,
              "mapped.css.map",
              "X-SourceMap header took effect"
            );
            seenSheets |= 1;
          } else if (/mapped2\.css/.test(sheet.href)) {
            is(
              sheet.sourceMapURL,
              "mapped2.css.map",
              "SourceMap header took effect"
            );
            seenSheets |= 2;
          } else {
            ok(false, "sheet does not have source map URL");
          }
        }

        is(seenSheets, 3, "seen all source-mapped sheets");
      });
    }
  );
});