Fix style updates to avoid unnecessary allocation (#2312)

* Fix style updates to avoid unnecessary allocation

- Drive-by: properly censor `cssFloat` -> css `float:` to match DOM
  behavior

* Document `style` syntax + custom property support. [skip ci]

* Add a missing test

* Add a few more descriptive comments.

* Update changelog [skip ci]

render/tests/test-createElement.js

@@ -45,6 +45,13 @@ o.spec("createElement", function() {
 
 		o(vnode.dom.style["--cssVar"]).equals("red")
 	})
+	o("censors cssFloat to float", function() {
+		var vnode = {tag: "a", attrs: {style: {cssFloat: "left"}}}
+
+		render(root, [vnode])
+
+		o(vnode.dom.style.float).equals("left")
+	})
 	o("creates children", function() {
 		var vnode = {tag: "div", children: [{tag: "a"}, {tag: "b"}]}
 		render(root, [vnode])