Paste a colour in any of the usual notations and get the other two. Three-digit shorthand is expanded by doubling each digit, which is what CSS does: #abc is #aabbcc, not #0a0b0c. Getting that wrong turns a light colour into a nearly black one, and since both are valid colours nothing complains. Unreadable input is named rather than silently treated as black.
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Each digit is doubled: #abc becomes #aabbcc. The other reading — padding each digit with a leading zero — gives #0a0b0c, which is almost black. Both are valid colours and neither the browser nor the eye will tell you which one you got until the design looks wrong.
Six-digit hex, three-digit hex, with or without the hash, and rgb() or rgba() with commas or spaces. The alpha value is read and ignored, since HEX and HSL as printed here describe the colour rather than its transparency.
Because HSL is stored as whole degrees and percentages here, and that is coarser than the 256 levels per channel that hex holds. Converting back can land a shade away. The hex and rgb values are exact; treat the HSL as the one for reasoning about hue and lightness rather than as a lossless copy.
It is refused, with a message saying what is accepted. Named colours would need the full CSS list, and quietly falling back to black — which is what an unparsed colour usually becomes — would be a wrong answer dressed as a right one.
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