Hex color codes, with the rules attached

Three unsigned bytes — red, green and blue — each written as two base-16 digits from 00 to ff. That gives 256 levels per channel and 16,777,216 codes in total, and it is exactly the same sRGB triple that rgb() carries in base 10, so #ff6347 and rgb(255, 99, 71) are one color written two ways. Hex is a notation, not a color model: nothing about the digits knows anything the three numbers do not.

Only when all three pairs happen to repeat their digit, which #663399 does and #663398 does not. The rule works outward rather than inward: a parser reading #639 writes each digit twice to reach #663399, so a pair such as 98 has no short form at all and the code stays at six digits. Of the 148 CSS keywords only 13 survive the test — among them red, white, black, lime, cyan and rebeccapurple — while all 216 web-safe codes survive it, since 00, 33, 66, 99, cc and ff are repeated digits by construction.

No — hex digits are case-insensitive in CSS, in SVG and in every browser parser, so the two are the same declaration. The difference is social: Figma, Sketch and most native color dialogs copy uppercase, Prettier and the Tailwind config print lowercase, and a codebase that mixes the two produces diff noise every time somebody re-copies a value out of a design tool. Pick one and normalize on paste, which is what the uppercase row on this page is for.

The trailing pair carries opacity, encoded the same way the color channels are — 00 through ff — so #66339980 is rebeccapurple at half strength. The trap is that the pair is not a percentage: 50% of 255 is 127.5, which rounds to 128 and prints as 80, not 7f, and 99 reads as 60% rather than 99%. The form arrived with CSS Color Module Level 4 and shipped in Firefox 49 and Safari 9.1 in 2016, Chrome 62 in 2017 and Edge only after it moved to Chromium in 2020; IE11 never accepted it, so a build that still lists IE11 needs rgba() instead.

Because the keyword list is two lists welded together. HTML 4.01 defined 16 VGA colors in which gray is #808080, while X11 had its own set in which gray was #bebebe and darkgray was #a9a9a9; CSS Color 3 kept the HTML meaning of gray and imported X11's darkgray unchanged, leaving #a9a9a9 sitting above #808080. The same merge is why green is #008000 while X11's bright green came in under the name lime, and why the seven gray keywords each answer to a grey spelling as well — nine of the 148 names are duplicate codes once aqua/cyan and fuchsia/magenta are counted, so the list covers 139 distinct colors.

Not for a normal stylesheet — the palette solved dithering on displays limited to 256 colors, which stopped being a constraint around 2000. It is still a useful grid for pixel art, for 8-bit and indexed-color export targets, and for retro palettes, which is why the chart keeps it on its own tab rather than mixed in. Worth knowing before you rely on it: a 2000 survey of the palette found only 22 of the 216 rendered identically across the Mac and Windows system palettes, so even at the time it was less safe than the name promised.

No — the chart is a static array compiled into the page and every lookup runs in your tab, so the page makes no request after it loads no matter how many codes you paste. That includes the export: Copy as CSS and Download .css build the :root block from the tiles currently on screen using a Blob created in the browser, so a client's brand palette never becomes a server log entry.