LibWeb: Make resolved serialization of CSS display prefer short form

Although we translate e.g `block` to `block flow` for internal use in
the engine, CSS-DISPLAY-3 tells us to use the short form in
serializations for compatibility reasons.

This adds 9 points to our score on https://html5test.com/ :^)

Tests/LibWeb/Text/input/include.js

@@ -0,0 +1,15 @@
+var __outputElement = null;
+
+function println(s) {
+    __outputElement.appendChild(document.createTextNode(s + "\n"));
+}
+
+document.addEventListener("DOMContentLoaded", function () {
+    __outputElement = document.createElement("pre");
+    __outputElement.setAttribute("id", "out");
+    document.body.appendChild(__outputElement);
+});
+
+function test(f) {
+    document.addEventListener("DOMContentLoaded", f);
+}