Hex representation of a color with alpha channel?

Question

Is there a W3 or any other noteworthy standard on how to represent a color (including alpha channel) in hex format?

Is it #RGBA or #ARGB?

Answer 1

In CSS 3, to quote from the spec, "there is no hexadecimal notation for an RGBA value" (see CSS Level 3 spec). Instead you can the use rgba() functional notation with decimals or percentages, e.g. rgba(255, 0, 0, 0.5) would be 50% transparent red. RGB channels are 0-255 or 0%-100%, alpha is 0-1.

In CSS 4*, you can specify the alpha channel using the 7th and 8th characters of an 8 digit hex colour, or 4th character of a 4 digit hex colour (see CSS Level 4 spec*)

As of May 2019, >80% of users can be expected to understand the #RGBA format

• Firefox has supported this syntax since Firefox 49 (Mozilla bug 567283).
• Safari has supported this syntax since Safari 10.
• Chrome has supported this syntax since Chrome 62. For earlier versions you could enable experimental web features to use this syntax. See Chromium Issue 618472 and Webkit bug 150853.
• Android Apps that target Android P or newer can use this syntax
• Opera supports this syntax in Opera 52 (or Opera 39 when experimental web features are enabled).
• IE 11 and EdgeHTML 18 (Edge 44) do not support this syntax. Chromium-based versions of Edge will support this syntax.

Up to date browser support information is available on CanIUse.com

*Technically still in draft, but given the browser support this is unlikely to be changed.