Remove comments from the *built* CSS files

The old pre-processor used for CSS, and HTML, files leaves comments intact which unnecessarily contributes to the overall size of the *built* CSS files (note that the built JavaScript files don't include comments).
Rather than trying to "hack" comment removal into the pre-processor it seems easier to use a PostCSS plugin instead. The one potential issue is that it also affects *some* whitespaces, and it's not clear to me if this'll work with the various CSS-related tests that run in mozilla-central.

Please refer to https://www.npmjs.com/package/postcss-discard-comments for additional information.

gulpfile.mjs

@@ -27,6 +27,7 @@ import { mkdirp } from "mkdirp";
 import path from "path";
 import postcss from "gulp-postcss";
 import postcssDirPseudoClass from "postcss-dir-pseudo-class";
+import postcssDiscardComments from "postcss-discard-comments";
 import postcssNesting from "postcss-nesting";
 import { preprocessPDFJSCode } from "./external/builder/preprocessor2.mjs";
 import rename from "gulp-rename";
@@ -934,17 +935,26 @@ gulp.task("cmaps", async function () {
 function preprocessCSS(source, defines) {
   const outName = getTempFile("~preprocess", ".css");
   builder.preprocess(source, outName, defines);
-  let out = fs.readFileSync(outName).toString();
+  const out = fs.readFileSync(outName).toString();
   fs.unlinkSync(outName);
 
-  // Strip out all license headers in the middle.
-  const reg = /\n\/\* Copyright(.|\n)*?Mozilla Foundation(.|\n)*?\*\//g;
-  out = out.replaceAll(reg, "");
-
   const i = source.lastIndexOf("/");
   return createStringSource(source.substr(i + 1), out);
 }
 
+function discardCommentsCSS() {
+  let copyrightNum = 0;
+
+  function remove(comment) {
+    // Remove all comments, except the *first* license header.
+    if (comment.startsWith("Copyright") && copyrightNum++ === 0) {
+      return false;
+    }
+    return true;
+  }
+  return postcssDiscardComments({ remove });
+}
+
 function preprocessHTML(source, defines) {
   const outName = getTempFile("~preprocess", ".html");
   builder.preprocess(source, outName, defines);
@@ -982,6 +992,7 @@ function buildGeneric(defines, dir) {
       .pipe(
         postcss([
           postcssDirPseudoClass(),
+          discardCommentsCSS(),
           postcssNesting(),
           autoprefixer(AUTOPREFIXER_CONFIG),
         ])
@@ -1069,6 +1080,7 @@ function buildComponents(defines, dir) {
       .pipe(
         postcss([
           postcssDirPseudoClass(),
+          discardCommentsCSS(),
           postcssNesting(),
           autoprefixer(AUTOPREFIXER_CONFIG),
         ])
@@ -1165,6 +1177,7 @@ function buildMinified(defines, dir) {
       .pipe(
         postcss([
           postcssDirPseudoClass(),
+          discardCommentsCSS(),
           postcssNesting(),
           autoprefixer(AUTOPREFIXER_CONFIG),
         ])
@@ -1411,12 +1424,22 @@ gulp.task(
         ),
 
         preprocessCSS("web/viewer.css", defines)
-          .pipe(postcss([autoprefixer(MOZCENTRAL_AUTOPREFIXER_CONFIG)]))
+          .pipe(
+            postcss([
+              discardCommentsCSS(),
+              autoprefixer(MOZCENTRAL_AUTOPREFIXER_CONFIG),
+            ])
+          )
           .pipe(replaceMozcentralCSS())
           .pipe(gulp.dest(MOZCENTRAL_CONTENT_DIR + "web")),
 
         preprocessCSS("web/viewer-geckoview.css", gvDefines)
-          .pipe(postcss([autoprefixer(MOZCENTRAL_AUTOPREFIXER_CONFIG)]))
+          .pipe(
+            postcss([
+              discardCommentsCSS(),
+              autoprefixer(MOZCENTRAL_AUTOPREFIXER_CONFIG),
+            ])
+          )
           .pipe(replaceMozcentralCSS())
           .pipe(gulp.dest(MOZCENTRAL_CONTENT_DIR + "web")),
 
@@ -1509,6 +1532,7 @@ gulp.task(
           .pipe(
             postcss([
               postcssDirPseudoClass(),
+              discardCommentsCSS(),
               postcssNesting(),
               autoprefixer(AUTOPREFIXER_CONFIG),
             ])

package-lock.json

@@ -48,6 +48,7 @@
         "pngjs": "^7.0.0",
         "postcss": "^8.4.30",
         "postcss-dir-pseudo-class": "^8.0.0",
+        "postcss-discard-comments": "^6.0.0",
         "postcss-nesting": "^12.0.1",
         "prettier": "^3.0.3",
         "puppeteer": "^21.2.1",
@@ -16768,6 +16769,18 @@
         "postcss": "^8.4"
       }
     },
+    "node_modules/postcss-discard-comments": {
+      "version": "6.0.0",
+      "resolved": "https://registry.npmjs.org/postcss-discard-comments/-/postcss-discard-comments-6.0.0.tgz",
+      "integrity": "sha512-p2skSGqzPMZkEQvJsgnkBhCn8gI7NzRH2683EEjrIkoMiwRELx68yoUJ3q3DGSGuQ8Ug9Gsn+OuDr46yfO+eFw==",
+      "dev": true,
+      "engines": {
+        "node": "^14 || ^16 || >=18.0"
+      },
+      "peerDependencies": {
+        "postcss": "^8.2.15"
+      }
+    },
     "node_modules/postcss-load-config": {
       "version": "3.1.0",
       "resolved": "https://registry.npmjs.org/postcss-load-config/-/postcss-load-config-3.1.0.tgz",

package.json

@@ -41,6 +41,7 @@
     "pngjs": "^7.0.0",
     "postcss": "^8.4.30",
     "postcss-dir-pseudo-class": "^8.0.0",
+    "postcss-discard-comments": "^6.0.0",
     "postcss-nesting": "^12.0.1",
     "prettier": "^3.0.3",
     "puppeteer": "^21.2.1",