Hexadecimal color notation gives CSS a compact way to specify a color. A six-digit value contains pairs of red, green, and blue components. Each pair ranges from 00 to FF; hexadecimal uses A–F for values ten through fifteen. section 11.2.2
Read the digits in pairs
#80c090 means 80 red, c0 green, and 90 blue—128, 192, and 144 in decimal. An eight-digit value adds an alpha pair: #80c09088 is semitransparent, with alpha ranging from 00 (fully transparent) to FF (fully opaque). Three-digit shorthand doubles each digit, so #c90 equals #cc9900; four-digit shorthand also includes alpha. section 11.2.2
aside {
background-color: #333333; #1
background-color: #333333aa; #2
}
The aside first receives an opaque background, then an eight-digit color with partial transparency. A browser that understands both uses the later declaration; one that does not understand eight-digit hex ignores it and keeps the opaque fallback. section 1.5.1
Hex is concise, but a value such as #076448 is difficult to identify by sight. Like rgb(), it represents only colors in the sRGB gamut, not the wider range available through newer color notations. section 11.2.1 section 11.2.2
