Switch to using `<link>` elements for stylesheets in shadow DOM

When Shadow DOM was first considered for use in the client `<link
rel="stylesheet">` was not widely supported as a way to load external styles into
shadow roots. This was resolved in the spec in https://github.com/whatwg/html/pull/1572
and has now long been supported by all browsers.

Using `<link>` simplifies the code and also avoids a possible problem with sites
that have a strict CSP policy that disallows inline styles. In the
browser extension context a `<link>` can still work in that scenario as long as the
stylesheet is loaded from a `chrome-extension://` URL.

src/annotator/util/shadow-root.js

@@ -10,20 +10,10 @@ function loadStyles(shadowRoot) {
     return;
   }
 
-  // Stylesheet <link> elements are inert inside shadow roots [1]. Until
-  // Shadow DOM implementations support external stylesheets [2], grab the
-  // relevant CSS files from the current page and `@import` them.
-  //
-  // [1] http://stackoverflow.com/questions/27746590
-  // [2] https://github.com/w3c/webcomponents/issues/530
-  //
-  // This will unfortunately break if the page blocks inline stylesheets via
-  // CSP, but that appears to be rare and if this happens, the user will still
-  // get a usable adder, albeit one that uses browser default styles for the
-  // toolbar.
-  const styleEl = document.createElement('style');
-  styleEl.textContent = `@import "${url}";`;
-  shadowRoot.appendChild(styleEl);
+  const linkEl = document.createElement('link');
+  linkEl.rel = 'stylesheet';
+  linkEl.href = url;
+  shadowRoot.appendChild(linkEl);
 }
 
 /**

src/annotator/util/test/shadow-root-test.js

@@ -32,9 +32,9 @@ describe('annotator/util/shadow-root', () => {
     it('injects stylesheets into the shadow root', () => {
       createShadowRoot(container);
 
-      const styleEl = container.shadowRoot.querySelector('style');
-      assert.ok(styleEl);
-      assert.match(styleEl.textContent, /@import ".*annotator\.css.*"/);
+      const linkEl = container.shadowRoot.querySelector('link[rel=stylesheet]');
+      assert.ok(linkEl);
+      assert.include(linkEl.href, 'annotator.css');
     });
 
     it('applies the applyFocusVisiblePolyfill if exists', () => {
@@ -52,8 +52,8 @@ describe('annotator/util/shadow-root', () => {
 
       createShadowRoot(container);
 
-      const styleEl = container.shadowRoot.querySelector('style');
-      assert.isNull(styleEl);
+      const linkEl = container.shadowRoot.querySelector('link[rel=stylesheet]');
+      assert.isNull(linkEl);
       link.setAttribute('rel', 'stylesheet');
     });
   });