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.
display:none
means that the tag in question will not appear on the page at all (although you can still interact with it through the dom). There will be no space allocated for it between the other tags.
visibility:hidden
means that unlike display:none
, the tag is not visible, but space is allocated for it on the page. The tag is rendered, it just isn't seen on the page.
For example:
test | <span style="[style-tag-value]">Appropriate style in this tag</span> | test
Replacing [style-tag-value]
with display:none
results in:
test | | test
Replacing [style-tag-value]
with visibility:hidden
results in:
test | | test
Best Answer
If you don't need to support IE9 or lower, you can use flexbox freely, and don't need to use floated layouts.
It's worth noting that today, the use of floated elements for layout is getting more and more discouraged with the use of better alternatives.
display: inline-block
- BetterFlexbox is supported from Firefox 18, Chrome 21, Opera 12.10, and Internet Explorer 10, Safari 6.1 (including Mobile Safari) and Android's default browser 4.4.
For a detailed browser list see: https://caniuse.com/flexbox.
(Perhaps once its position is established completely, it may be the absolutely recommended way of laying out elements.)
A clearfix is a way for an element to automatically clear its child elements, so that you don't need to add additional markup. It's generally used in float layouts where elements are floated to be stacked horizontally.
The clearfix is a way to combat the zero-height container problem for floated elements
A clearfix is performed as follows:
Or, if you don't require IE<8 support, the following is fine too:
Normally you would need to do something as follows:
With clearfix, you only need the following:
Read about it in this article - by Chris Coyer @ CSS-Tricks