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.
Using
$("a").attr("href", "http://www.google.com/")
will modify the href of all hyperlinks to point to Google. You probably want a somewhat more refined selector though. For instance, if you have a mix of link source (hyperlink) and link target (a.k.a. "anchor") anchor tags:
<a name="MyLinks"></a>
<a href="http://www.codeproject.com/">The CodeProject</a>
...Then you probably don't want to accidentally add href
attributes to them. For safety then, we can specify that our selector will only match <a>
tags with an existing href
attribute:
$("a[href]") //...
Of course, you'll probably have something more interesting in mind. If you want to match an anchor with a specific existing href
, you might use something like this:
$("a[href='http://www.google.com/']").attr('href', 'http://www.live.com/')
This will find links where the href
exactly matches the string http://www.google.com/
. A more involved task might be matching, then updating only part of the href
:
$("a[href^='http://stackoverflow.com']")
.each(function()
{
this.href = this.href.replace(/^http:\/\/beta\.stackoverflow\.com/,
"http://stackoverflow.com");
});
The first part selects only links where the href starts with http://stackoverflow.com
. Then, a function is defined that uses a simple regular expression to replace this part of the URL with a new one. Note the flexibility this gives you - any sort of modification to the link could be done here.
Best Answer
Most people are used to HTML 4/XHTML and before, where the
type
attribute is required for these elements.In regards to HTML 5, these are indeed optional and the spec gives a default, depending on the element.
For the
script
tag, this defaults totext/javascript
:For the
style
tag, this defaults totext/css
:So, not needed, as you stated. However, browser support and server setups can't always be relied on - being explicit is a good idea as it avoids such problems.
And of course, not all browsers out there support HTML 5 - those that don't will use an earlier version where the attribute is required and your javascript/css might not get parsed in such browsers, meaning you end up with no CSS or javascript on older browsers, when a simple solution for backwards compatibility is to add the attribute.