Example 1

When appearance is specified later than a custom value for a vendor prefixed equivalent, it includes the vendor prefixed equivalent into vendor prefixing. (As CSS reads from top to bottom it will take the last one)

.foo {
  -webkit-appearance: none;
  appearance: textfield;
}

Current

.foo {
  -webkit-appearance: none;
  -webkit-appearance: textfield;
  -moz-appearance: textfield;
  appearance: textfield;
}

Sandbox

Expected

.foo {
  -webkit-appearance: none;
  -moz-appearance: textfield;
  appearance: textfield;
}

Example 2

When a vendor prefixed property is specified later than the default one, all of the added prefixed properties become the value of the last one

.bar {
  appearance: textfield;
  -webkit-appearance: none;
}

Current

.bar {
  -webkit-appearance: none;
  -moz-appearance: none;
  appearance: none;
}

Sandbox

Expected

.bar {
  -moz-appearance: textfield;
  appearance: textfield;
  -webkit-appearance: none;
}

Note

Currently it's impossible to have a custom value for 1 specific browser, as the last one is leading with all options
I would expect a matching result as with the PostCSS autoprefixer plugin