Implement a fallback if CSS blending is not supported

Implement a fallback for browsers that don't support the CSS
`mix-blend-mode` property (IE 11, Edge < 79) by merging overlapping
highlights into a single layer with uniform opacity. This prevents
overlapping highlights from affecting readability since highlights are
blended with the content underneath using normal blending in this case.

src/annotator/highlighter.js

 const SVG_NAMESPACE = 'http://www.w3.org/2000/svg';
 
+function isCSSPropertySupported(property, value) {
+  if (typeof CSS !== 'function' || typeof CSS.supports !== 'function') {
+    /* istanbul ignore next */
+    return false;
+  }
+  return CSS.supports(property, value);
+}
+
 /**
  * Polyfill for `element.closest(selector)`, only needed for IE 11.
  */
@@ -73,6 +81,11 @@ function drawHighlightsAbovePdfCanvas(highlightEl) {
     '.hypothesis-highlight-layer'
   );
 
+  const isCssBlendSupported = isCSSPropertySupported(
+    'mix-blend-mode',
+    'multiply'
+  );
+
   if (!svgHighlightLayer) {
     // Create SVG layer. This must be in the same stacking context as
     // the canvas so that CSS `mix-blend-mode` can be used to control how SVG
@@ -91,15 +104,20 @@ function drawHighlightsAbovePdfCanvas(highlightEl) {
     svgStyle.width = '100%';
     svgStyle.height = '100%';
 
+    if (isCssBlendSupported) {
       // Use multiply blending so that highlights drawn on top of text darken it
       // rather than making it lighter. This improves contrast and thus readability
-    // of highlighted text. This choice optimizes for dark text on a light
-    // background, as the most common case.
+      // of highlighted text, especially for overlapping highlights.
       //
-    // Browsers which don't support the `mix-blend-mode` property (IE 11, Edge < 79)
-    // will use "normal" blending, which is still usable but has reduced contrast,
-    // especially for overlapping highlights.
+      // This choice optimizes for the common case of dark text on a light background.
       svgStyle.mixBlendMode = 'multiply';
+    } else {
+      // For older browsers (IE 11, Edge < 79) we draw all the highlights as
+      // opaque and then make the entire highlight layer transparent. This means
+      // that there is no visual indication of whether text has one or multiple
+      // highlights, but it preserves readability.
+      svgStyle.opacity = 0.3;
+    }
   }
 
   const canvasRect = canvasEl.getBoundingClientRect();
@@ -111,7 +129,13 @@ function drawHighlightsAbovePdfCanvas(highlightEl) {
   rect.setAttribute('y', highlightRect.top - canvasRect.top);
   rect.setAttribute('width', highlightRect.width);
   rect.setAttribute('height', highlightRect.height);
+
+  if (isCssBlendSupported) {
     rect.setAttribute('class', 'hypothesis-svg-highlight');
+  } else {
+    rect.setAttribute('class', 'hypothesis-svg-highlight is-opaque');
+  }
+
   svgHighlightLayer.appendChild(rect);
 
   return rect;

src/annotator/test/highlighter-test.js

@@ -221,6 +221,53 @@ describe('annotator/highlighter', () => {
         assert.isNull(container.querySelector('rect'));
         assert.notOk(highlight.svgHighlight);
       });
+
+      describe('CSS blend mode support testing', () => {
+        beforeEach(() => {
+          sinon.stub(CSS, 'supports');
+        });
+
+        afterEach(() => {
+          CSS.supports.restore();
+        });
+
+        it('renders highlights when mix-blend-mode is supported', () => {
+          const container = document.createElement('div');
+          render(<PdfPage />, container);
+          CSS.supports.withArgs('mix-blend-mode', 'multiply').returns(true);
+
+          highlightPdfRange(container);
+
+          // When mix blending is available, the highlight layer has default
+          // opacity and highlight rects are transparent.
+          const highlightLayer = container.querySelector(
+            '.hypothesis-highlight-layer'
+          );
+          assert.equal(highlightLayer.style.opacity, '');
+          const rect = container.querySelector('rect');
+          assert.equal(rect.getAttribute('class'), 'hypothesis-svg-highlight');
+        });
+
+        it('renders highlights when mix-blend-mode is not supported', () => {
+          const container = document.createElement('div');
+          render(<PdfPage />, container);
+          CSS.supports.withArgs('mix-blend-mode', 'multiply').returns(false);
+
+          highlightPdfRange(container);
+
+          // When mix blending is not available, highlight rects are opaque and
+          // the entire highlight layer is transparent.
+          const highlightLayer = container.querySelector(
+            '.hypothesis-highlight-layer'
+          );
+          assert.equal(highlightLayer.style.opacity, '0.3');
+          const rect = container.querySelector('rect');
+          assert.include(
+            rect.getAttribute('class'),
+            'hypothesis-svg-highlight is-opaque'
+          );
+        });
+      });
     });
   });
 

src/styles/annotator/highlights.scss

@@ -27,6 +27,10 @@
 .hypothesis-highlights-always-on {
   .hypothesis-svg-highlight {
     fill: var.$highlight-color;
+
+    &.is-opaque {
+      fill: yellow;
+    }
   }
 
   .hypothesis-highlight {