diff --git a/resources.whatwg.org/standard.css b/resources.whatwg.org/standard.css
index 61d835ae8..9901270e4 100644
--- a/resources.whatwg.org/standard.css
+++ b/resources.whatwg.org/standard.css
@@ -571,3 +571,52 @@ pre > code.css, pre.highlight.lang-css {
 .wpt-tests-margin.wrapped > :not(input) {
   display: none;
 }
+
+/* Dfn panels */
+.dfn-panel {
+    display: inline-block;
+    position: absolute;
+    z-index: 35;
+    height: auto;
+    width: -webkit-fit-content;
+    width: fit-content;
+    max-width: 300px;
+    max-height: 500px;
+    overflow: auto;
+    padding: 0.5em 0.75em;
+    font: small Helvetica Neue, sans-serif, Droid Sans Fallback;
+    background: #ddd;
+    color: black;
+    border: outset 0.2em;
+    cursor: initial;
+}
+.dfn-panel:not(.on) { display: none; }
+.dfn-panel * { margin: 0; padding: 0; text-indent: 0; }
+.dfn-panel > b { display: block; }
+.dfn-panel a { color: black; }
+.dfn-panel a:not(:hover) { text-decoration: none !important; border-bottom: none !important; }
+.dfn-panel p:not(.spec-link) { font-weight: bolder; }
+.dfn-panel > b + b, .dfn-panel > * + p { margin-top: 0.25em; }
+.dfn-panel ul { padding: 0; }
+.dfn-panel li { list-style: inside; }
+.dfn-panel.activated {
+    position: fixed;
+    left: .5em;
+    bottom: 2em;
+    margin: 0 auto;
+    max-width: calc(100vw - 1.5em - .4em - .5em);
+    max-height: 30vh;
+}
+
+/* Elements that trigger a dfn panel when clicked.
+   The dfn-paneled class is specific to Bikeshed, while dfnEnabled
+   identifies builds of the HTML spec that support dfn panels. */
+.dfn-paneled,
+body.dfnEnabled dfn,
+body.dfnEnabled h2[data-dfn-type],
+body.dfnEnabled h3[data-dfn-type],
+body.dfnEnabled h4[data-dfn-type],
+body.dfnEnabled h5[data-dfn-type],
+body.dfnEnabled h6[data-dfn-type] {
+  cursor: pointer;
+}