[Optimization] Initial background

[Semigradsky]

Before:

    .a {
        background: none
    }
    .b {
        background: none center
    }
    .c {
        background: url("test.png") transparent none repeat scroll
    }

After:

.a{background:#0000}.b{background:50% 50%}.c{background:url("test.png") transparent none repeat scroll}

For .a we even have deoptimization (none vs #0000).

Possible optimised output (from csso):

.a{background:0 0}.b{background:center}.c{background:url(test.png)}

[devongovett]

background:url("test.png") transparent none repeat scroll is invalid (if you remove the none it works), which is why it doesn't minify (Parcel CSS simply passes through properties that cannot be parsed correctly). Changing it how csso does changes the behavior, which we'd like to avoid.

The others look like good optimizations though.