Adding to Tim's answer:
#search:placeholder-shown {
// show background image, I like svg
// when using svg, do not use HEX for colour; you can use rbg/a instead
// also notice the single quotes
background-image url('data:image/svg+xml; utf8, <svg>... <g fill="grey"...</svg>')
// other background props
}
#search:not(:placeholder-shown) { background-image: none;}