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Why does HTML think "chucknorris" is a color?

Why do certain random strings produce colors when entered as background colors in HTML?

For example, bgcolor="chucknorris" produces a red background:

<body bgcolor="chucknorris"> test </body>

Conversely, bgcolor="chucknorr" produces a yellow background:

<body bgcolor="chucknorr"> test </body>

This holds true across various browsers and platforms. What’s going on here?

Answer*

The [WHATWG HTML specification has the exact algorithm][1] for parsing a legacy color value.

The code Netscape Classic used for parsing color strings is open source: *[netscape/lib/layout/layimage.c][2]*.

For example, notice that each character is parsed as a hex digit and then is shifted into a 32-bit integer **without checking for overflow**. Only eight hex digits fit into a 32-bit integer, which is why only the last 8 characters are considered. After parsing the hex digits into 32-bit integers, they are then truncated into 8-bit integers by dividing them by 16 until they fit into 8-bit, which is why leading zeros are ignored.

This code does not exactly match what is defined in the spec, but the only difference there is a few lines of code. I think it is [these lines][3] that were added (in Netscape 4):

```c
if (bytes_per_val > 4)
{
	bytes_per_val = 4;
}
```


 [1]: https://html.spec.whatwg.org/multipage/infrastructure.html#rules-for-parsing-a-legacy-colour-value
 [2]: https://github.com/zii/netscape/blob/master/lib/layout/layimage.c#L150
 [3]: https://github.com/zii/netscape/blob/master/lib/layout/layimage.c#L205

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The first link is (effectively) broken. There doesn't seem to be a section with a headline of "Rules for parsing a legacy colour value" (or similar). Though there is a similar page anchor, "legacy-extract-an-encoding"

Peter Mortensen
– Peter Mortensen

2022年11月30日 22:23:35 +00:00
Commented Nov 30, 2022 at 22:23
This is probably the correct link: html.spec.whatwg.org/multipage/…

n.r.
– n.r.

2023年04月27日 06:48:45 +00:00
Commented Apr 27, 2023 at 6:48

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Why does HTML think "chucknorris" is a color?

Answer*