It's not recommended to group selectors that are meant to work with different browsers. For example, styling an input's placeholder needs multiple selectors per the browser. If we group the selectors, the entire rule will be invalid, according to w3c.

/* Don't do this, please */
input::-webkit-input-placeholder,
input:-moz-placeholder {
    color: #222;
}

Instead, do this.

input::-webkit-input-placeholder {
    color: #222;
}
input:-moz-placeholder {
    color: #222;
}