What is the standard way to eliminate unsupported CSS expressions?

Question

What does the CSS standard say about unsupported expressions? How should a browser deal with them? How do actual browser implementations cope with them?

I am implementing CSS property optimizer (for the minifier project), and we want to leave unused CSS backups. Our goal is to optimize the CSS as much as possible, but in such a way that it displays exactly the same as the original .
That’s why it’s important for me to understand how this works.

Simple properties

For simple properties, this is very simple.
Say we have this:

div {
    color: #f00;
    color: rgba(1,2,3,.4);
}

In this case, if the browser does not support rgba, the first ad wins #f00. There are no questions here.

Shorthands

, ?
:

div {
    background: #f00;
    background: rgba(1,2,3,.4);
}

, rgba?. , background: background: <color> <image> <repeat> <attachment> <position>;, 5 , ; , 5 . :

• , .
• , rgba(...) background-image, , , background-color
• , rgba(...) a background-color , , #f00

, , :

div {
    background: #fff url(...) no-repeat;
    background: rgba(1,2,3,.4) linear-gradient(...) repeat-y;
}

CSS,...

• rgba?
• linear-gradient?
• repeat-y?
• ?
• ?

Answer 1

, , .

Answer 2

4.2 CSS2.1 , , , , :

• . . :

  img { float: left }       /* correct CSS 2.1 */
  img { float: left here }  /* "here" is not a value of 'float' */
  img { background: "red" } /* keywords cannot be quoted */
  img { border-width: 3 }   /* a unit must be specified for length values */
  

  CSS 2.1 , :

  img { float: left }
  img { }
  img { }
  img { }
  

  , CSS, .

, background, .

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