For HTML 4, the answer is technically:
ID and NAME tokens must begin with a letter ([A-Za-z]) and may be followed by any number of letters, digits ([0-9]), hyphens ("-"), underscores ("_"), colons (":"), and periods (".").
HTML 5 is even more permissive, saying only that an id must contain at least one character and may not contain any space characters.
The id attribute is case sensitive in XHTML.
As a purely practical matter, you may want to avoid certain characters. Periods, colons and '#' have special meaning in CSS selectors, so you will have to escape those characters using a backslash in CSS or a double backslash in a selector string passed to jQuery. Think about how often you will have to escape a character in your stylesheets or code before you go crazy with periods and colons in ids.
For example, the HTML declaration <div id="first.name"></div>
is valid. You can select that element in CSS as #first\.name
and in jQuery like so: $('#first\\.name').
But if you forget the backslash, $('#first.name')
, you will have a perfectly valid selector looking for an element with id first
and also having class name
. This is a bug that is easy to overlook. You might be happier in the long run choosing the id first-name
(a hyphen rather than a period), instead.
You can simplify your development tasks by strictly sticking to a naming convention. For example, if you limit yourself entirely to lower-case characters and always separate words with either hyphens or underscores (but not both, pick one and never use the other), then you have an easy-to-remember pattern. You will never wonder "was it firstName
or FirstName
?" because you will always know that you should type first_name
. Prefer camel case? Then limit yourself to that, no hyphens or underscores, and always, consistently use either upper-case or lower-case for the first character, don't mix them.
A now very obscure problem was that at least one browser, Netscape 6, incorrectly treated id attribute values as case-sensitive. That meant that if you had typed id="firstName"
in your HTML (lower-case 'f') and #FirstName { color: red }
in your CSS (upper-case 'F'), that buggy browser would have failed to set the element's color to red. At the time of this edit, April 2015, I hope you aren't being asked to support Netscape 6. Consider this a historical footnote.
Best Answer
It’s a holdover from the Netscape days:
It is from the blog post A little rant about Microsoft Internet Explorer's color parsing which covers it in great detail, including varying lengths of color values, etc.
If we apply the rules in turn from the blog post, we get the following:
Replace all nonvalid hexadecimal characters with 0’s:
Pad out to the next total number of characters divisible by 3 (11 → 12):
Split into three equal groups, with each component representing the corresponding colour component of an RGB colour:
Truncate each of the arguments from the right down to two characters.
Which, finally, gives the following result:
Here’s an example demonstrating the
bgcolor
attribute in action, to produce this “amazing” colour swatch:This also answers the other part of the question: Why does
bgcolor="chucknorr"
produce a yellow colour? Well, if we apply the rules, the string is:Which gives a light yellow gold colour. As the string starts off as 9 characters, we keep the second ‘C’ this time around, hence it ends up in the final colour value.
I originally encountered this when someone pointed out that you could do
color="crap"
and, well, it comes out brown.