Fix #14432: Always return string from .css("z-index"). Close gh-1395.
(cherry picked from commit 5ce4b06c285bd8cf52eaff0f39e0b9192a927873)

Conflicts:

	src/css/curCSS.js
No ticket: Small curCSS size optimizations
(cherry picked from commit a339096d6a9d0313c2184de0d626e2c7e127955b)

Conflicts:

	src/css/curCSS.js
No ticket: Test cleanup
(cherry picked from commit 8df348e9eb3681ff5f09b1e52b72573e2eee3105)

Conflicts:

	test/unit/css.js