Update highlights CSS to prepare for clustered-highlight styling

Refactor `highlights.scss` to configure highlight style and subsequently apply styles. Add CSS variables for use for dynamic styling of highlight clusters.

Update some comment wording in `gulpfile` for clarity.

gulpfile.mjs

@@ -36,15 +36,12 @@ gulp.task('build-sidebar-tailwind-css', () =>
   )
 );
 
-// These CSS source files should not use Tailwind, as they are used in
-// third-party contexts.
-// TODO: Remove `tailwindConfig` after updating `sass` task in `frontend-build`.
-// Setting it is necessary currently to suppress a build warning.
 gulp.task('build-standalone-css', () =>
   buildCSS(
     [
-      // other styles used by annotator (standalone)
+      // styles processed by tailwind, used by annotator
       './src/styles/annotator/highlights.scss',
+      // other styles used by annotator (standalone)
       './src/styles/annotator/pdfjs-overrides.scss',
 
       // Vendor

src/styles/annotator/highlights.scss

-$color-highlight: rgba(255, 255, 60, 0.4);
-$color-highlight-second: rgba(206, 206, 60, 0.4);
-$color-highlight-focused: rgba(156, 230, 255, 0.5);
-
-// Hide content from sighted users but make it visible to screen readers.
-//
-// Resources:
-// - https://webaim.org/techniques/css/invisiblecontent/ (see "CSS clip")
-// - https://cloudfour.com/thinks/see-no-evil-hidden-content-and-accessibility/#showing-additional-content-for-screen-readers
-// - https://tailwindcss.com/docs/screen-readers#screen-reader-only-elements Tailwind's `sr-only` class
-@mixin screen-reader-only {
-  // Take the content out of the normal layout flow.
-  position: absolute;
-  width: 1px;
-  height: 1px;
-  white-space: nowrap;
-
-  // Visually hide the content and prevent it from interfering with mouse/touch
-  // text selection by clipping it to an empty box. Compared to moving content with
-  // `top`/`left` this is less likely to cause the browser to scroll to a
-  // different part of the page when the hidden text has screen-reader focus.
-  clip: rect(0 0 0 0);
-  overflow: hidden;
+:root {
+  // Default highlight styling configuration
+  --hypothesis-highlight-color: rgba(255, 255, 60, 0.4);
+  --hypothesis-highlight-focused-color: rgba(156, 230, 255, 0.5);
+  --hypothesis-highlight-blend-mode: normal;
+  --hypothesis-highlight-decoration: none;
+
+  --hypothesis-highlight-second-color: rgba(206, 206, 60, 0.4);
+  --hypothesis-highlight-third-color: transparent;
 }
 
-// SVG highlights when the "Show Highlights" toggle is turned off.
+// Configure highlight styling.
+
+// Map `--hypothesis-*` root values to local `--highlight-*` values
 .hypothesis-svg-highlight {
-  fill: transparent;
+  --highlight-color: var(--hypothesis-highlight-color);
+  --highlight-color-focused: var(--hypothesis-highlight-focused-color);
 }
 
-// `hypothesis-highlights-always-on` is a class that is toggled on the root
-// of the annotated document when highlights are enabled/disabled.
-.hypothesis-highlights-always-on {
-  .hypothesis-svg-highlight {
-    fill: $color-highlight;
+.hypothesis-highlight {
+  --highlight-color: var(--hypothesis-highlight-color);
+  --highlight-blend-mode: var(--hypothesis-highlight-blend-mode);
+  --highlight-decoration: var(--hypothesis-highlight-decoration);
+  --highlight-color-focused: var(--hypothesis-highlight-focused-color);
 
-    &.is-opaque {
-      fill: yellow;
-    }
+  & .hypothesis-highlight {
+    --highlight-color: var(--hypothesis-highlight-second-color);
 
-    &.is-focused {
-      fill: $color-highlight-focused;
+    .hypothesis-highlight {
+      // Highlights more than two levels deep are transparent by default.
+      --highlight-color: var(--hypothesis-highlight-third-color);
     }
   }
+}
 
-  .hypothesis-highlight {
-    background-color: $color-highlight;
+// Apply highlight styling.
 
+// Highlights are non-visible when .hypothesis-highlight-always-on class not present.
+.hypothesis-highlight {
+  background-color: transparent;
   // For PDFs, we still create highlight elements to wrap the text but the
   // highlight effect is created by another element.
   &.is-transparent {
-      background-color: transparent;
+    background-color: transparent !important;
   }
+}
+
+.hypothesis-svg-highlight {
+  fill: transparent;
+}
+
+// Apply styling using `--highlight-` values when highlights are visible
+
+.hypothesis-highlights-always-on .hypothesis-svg-highlight {
+  fill: var(--highlight-color);
+
+  &.is-opaque {
+    fill: yellow;
+  }
+
+  &.is-focused {
+    fill: var(--highlight-color-focused);
+  }
+}
+
+.hypothesis-highlights-always-on .hypothesis-highlight {
+  background-color: var(--highlight-color);
+  text-decoration: var(--highlight-decoration);
+  mix-blend-mode: var(--highlight-blend-mode);
 
   cursor: pointer;
 
   // Make highlights visible to screen readers.
   // See also - https://developer.paciellogroup.com/blog/2017/12/short-note-on-making-your-mark-more-accessible/.
   &::before {
-      @include screen-reader-only;
+    @apply sr-only;
 
     // nb. The leading/trailing spaces are intended to ensure the text is treated
     // as separate words by assistive technologies from the content before/after it.
     content: ' annotation start ';
   }
   &::after {
-      @include screen-reader-only;
+    @apply sr-only;
     content: ' annotation end ';
   }
+}
 
-    // Give a highlight inside a larger highlight a different color to stand out.
-    & .hypothesis-highlight {
-      background-color: $color-highlight-second;
-      &.is-transparent {
-        background-color: transparent;
-      }
-
-      // Limit the number of different highlight shades by making highlights
-      // that are 3+ levels deep transparent.
-      //
-      // This was historically done to improve readability in PDFs [1], but that
-      // is no longer an issue as in PDFs highlights are created by drawing
-      // SVGs on top of the <canvas>.
-      //
-      // [1] https://github.com/hypothesis/client/issues/1995.
-      & .hypothesis-highlight {
-        background-color: transparent;
-      }
-    }
+// Apply focused-highlight styling
 
+.hypothesis-highlights-always-on
+  .hypothesis-highlight.hypothesis-highlight-focused {
   // When an annotation card is hovered in the sidebar, the corresponding
   // highlights are shown with a "focused" color.
   &.hypothesis-highlight-focused {
-      background-color: $color-highlight-focused !important;
+    mix-blend-mode: normal !important;
+    background-color: var(--highlight-color-focused) !important;
+    text-decoration: none;
 
     .hypothesis-highlight {
       background-color: transparent !important;
     }
   }
-  }
 }
 
 // Placeholder element to which annotations for off-screen content in PDFs