Add styles for <wpt> output to standard.css

See whatwg/wattsi#90. Eventually we may use similar styles for <wpt> annotations in specs besides HTML; for now they are using Bikeshed's built-in styles. Unification is tracked at whatwg/meta#117.

resources.whatwg.org/standard.css

@@ -506,3 +506,48 @@ c-[vc] { color: #0077aa } /* Name.Variable.Class */
 c-[vg] { color: #0077aa } /* Name.Variable.Global */
 c-[vi] { color: #0077aa } /* Name.Variable.Instance */
 c-[il] { color: #000000 } /* Literal.Number.Integer.Long */
+
+.wpt-tests-margin {
+  background: hsl(290, 70%, 95%);
+  margin: 1em auto;
+  padding: .7em;
+  width: 136px;
+  font-size: 11px;
+  position: absolute;
+  right: 4.8px;
+  z-index: 9;
+  box-shadow: 0 0 3px #999
+}
+.wpt-tests-margin dl::before {
+  content: "Tests";
+  color: hsl(290, 70%, 30%);
+  text-transform: uppercase;
+}
+.wpt-tests-margin dt {
+  font-weight: normal;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  margin: 0;
+}
+.wpt-tests-margin dt:first-child {
+  margin-top: 2px;
+}
+.wpt-tests-margin dd {
+  margin-left: 20px;
+  line-height: 1em;
+  margin-bottom: 4px;
+}
+.wpt-tests-margin > input {
+  position: absolute;
+  left: 0;
+  top: 0;
+  width: 1em;
+  height: 1em;
+  border: none;
+  background: transparent;
+  padding: 0;
+  margin: 0;
+}
+.wpt-tests-margin.wrapped > :not(input) {
+  display: none;
+}