cmd/pkgserver/ui_test: add DOM reporter

cmd/pkgserver/ui_test/lib/ui.scss

@@ -0,0 +1,52 @@
+:root {
+    --bg: #d3d3d3;
+    --fg: black;
+}
+
+@media (prefers-color-scheme: dark) {
+    :root {
+        --bg: #2c2c2c;
+        --fg: ghostwhite;
+    }
+}
+
+html {
+    background-color: var(--bg);
+    color: var(--fg);
+}
+
+h1, p, summary, noscript {
+    font-family: sans-serif;
+}
+
+noscript {
+    font-size: 16pt;
+}
+
+.root {
+    margin: 1rem 0;
+}
+
+details.test-node {
+    margin-left: 1rem;
+    padding: 0.2rem 0.5rem;
+    border-left: 2px dashed var(--fg);
+    > summary {
+        cursor: pointer;
+    }
+    &.failure > summary::marker {
+        color: red;
+    }
+}
+
+p.test-desc {
+    margin: 0 0 0 1rem;
+    padding: 2px 0;
+    > pre {
+        margin: 0;
+    }
+}
+
+.italic {
+    font-style: italic;
+}