LibWeb: Implement appearance CSS property

This includes the "compat" values even though they are not strictly
necessary.

Userland/Libraries/LibWeb/CSS/Identifiers.json

@@ -74,6 +74,7 @@
   "bottom",
   "break-word",
   "browser",
+  "button",
   "capitalize",
   "cell",
   "center",
@@ -96,6 +97,7 @@
   "currentcolor",
   "cursive",
   "custom",
+  "checkbox",
   "dark",
   "dashed",
   "decimal",
@@ -160,12 +162,16 @@
   "lower-roman",
   "lowercase",
   "ltr",
+  "listbox",
   "medium",
   "middle",
   "minimal-ui",
   "monospace",
   "more",
   "move",
+  "menulist",
+  "meter",
+  "menulist-button",
   "n-resize",
   "ne-resize",
   "nesw-resize",
@@ -197,6 +203,8 @@
   "pre-wrap",
   "progress",
   "progressive",
+  "push-button",
+  "progress-bar",
   "rec2020",
   "reduce",
   "relative",
@@ -216,6 +224,7 @@
   "ruby-text",
   "ruby-text-container",
   "run-in",
+  "radio",
   "s-resize",
   "safe",
   "sans-serif",
@@ -245,6 +254,9 @@
   "subtractive",
   "super",
   "sw-resize",
+  "searchfield",
+  "slider-horizontal",
+  "square-button",
   "table",
   "table-caption",
   "table-cell",
@@ -260,6 +272,8 @@
   "thick",
   "thin",
   "top",
+  "textarea",
+  "textfield",
   "ui-monospace",
   "ui-rounded",
   "ui-sans-serif",