Properly copy/paste allowed HTML

app/assets/javascripts/copy_as_gfm.js.es6

@@ -91,6 +91,28 @@
         return text.trim();
       },
     },
+    SanitizationFilter: {
+      'br'(el, text) {
+        return '<br>';
+      },
+      'dl'(el, text) {
+        let lines = text.trim().split('\n');
+        // Add two spaces to the front of subsequent list items lines,
+        // or leave the line entirely blank.
+        lines = lines.map((s) => {
+          s = s.trim();
+          if (s.length === 0) return '';
+
+          return `  ${s}`;
+        });
+
+        return `<dl>\n${lines.join('\n')}\n</dl>`;
+      },
+      'sub, dt, dd, kbd, q, samp, var, ruby, rt, rp, abbr'(el, text) {
+        const tag = el.nodeName.toLowerCase();
+        return `<${tag}>${text}</${tag}>`;
+      },
+    },
     SyntaxHighlightFilter: {
       'pre.code.highlight'(el, text) {
         let lang = el.getAttribute('lang');

spec/features/copy_as_gfm_spec.rb

@@ -233,6 +233,37 @@ describe 'Copy as GFM', feature: true, js: true do
     end
   end
 
+  it 'supports SanitizationFilter' do
+    verify(
+      <<-GFM.strip_heredoc
+      BR: <br>
+
+      <sub>sub</sub>
+
+      <dl>
+        <dt>dt</dt>
+        <dd>dd</dd>
+      </dl>
+
+      <kbd>kbd</kbd>
+
+      <q>q</q>
+
+      <samp>samp</samp>
+
+      <var>var</var>
+
+      <ruby>ruby</ruby>
+
+      <rt>rt</rt>
+
+      <rp>rp</rp>
+
+      <abbr>abbr</abbr>
+      GFM
+    )
+  end
+
   it 'supports SyntaxHighlightFilter' do
     verify(
       <<-GFM.strip_heredoc,