Merging adjacent rules with *almost* the same declarations

README says:

Merging adjacent rules with the same selectors or declarations when it is safe to do so.

This works as advertised:

.foo {
    background-color: red;
}
.bar {
    background-color: red;
}

/* output */
.foo,.bar{background-color:red}

But I wonder, is there any reason not to merge partially-indentical rules?

.foo {
    background-color: red;
    padding: 20px;
}
.bar {
    background-color: red;
    padding: 40px;
}

/* actual output */
.foo{background-color:red;padding:20px}.bar{background-color:red;padding:40px}

/* better output? */
.foo,.bar{background-color:red}.foo{padding:20px}.bar{padding:40px}

/* or maybe even: */
.foo,.bar{background-color:red;padding:20px}.bar{padding:40px}

Or am I wrong to think that as long as the rules are adjacent, this should be safe to do?

(The gains aren't big in this simplified example, but we have CSS with long adjacent rules that differ in only one or two declarations, and it quickly adds up.)

The issue provides CSS examples but names no implementation files, tests, or entry points. First determine whether partially identical adjacent rules can be merged without changing CSS semantics, then establish the intended output and add regression coverage for the chosen behavior.