PROTOTYPE: Generic custom CSS property parsing

chrome/browser/resources/vr_shell/vr_shell_ui.css

Index: chrome/browser/resources/vr_shell/vr_shell_ui.css
diff --git a/chrome/browser/resources/vr_shell/vr_shell_ui.css b/chrome/browser/resources/vr_shell/vr_shell_ui.css
index aae8654fb3bd7ebc43478c78ac1f5663556ea87f..34ff48ad9715153ff45386d7315f39bf828f343c 100644
--- a/chrome/browser/resources/vr_shell/vr_shell_ui.css
+++ b/chrome/browser/resources/vr_shell/vr_shell_ui.css
@@ -21,6 +21,23 @@ html {
   margin: 2px;
 }
 
+#content-quad-element {
+  --sizeY: 1.6;
+  --sizeX: 2.5;
+  width: 50px;
+  height: 50px;
+  background-color: red;
+  --tranX: 0;
+  --tranY: 0;
+  --tranZ: -2.0;
+}
+
+#content-quad-element.menu {
+  --tranX: 0;
+  --tranY: 0;
+  --tranZ: -4.0;
+}
+
 .webvr-message-box {
   align-items: center;
   display: flex;
@@ -86,6 +103,12 @@ html {
   opacity: 0.8;
   transition: opacity 150ms ease-in-out;
   width: 96px;
+  --custom-var: "cat";
+}
+
+.round-button .button:hover {
+  background-color: red;
+  --custom-var: "dog";
 }
 
 .round-button .caption {