PROBLEM

What characters/symbols are allowed within CSS class selectors? we know that the following characters are invalid, but what characters are valid?
~ ! @ $ % ^ & * ( ) + = , . / ‘ ; : ” ? > < [ ] \ { } | ` #

SOLUTION 1:

• There are no CSS classes.
• The question logically splits to two questions:
• can you start a class name with a digit in HTML, and if it does, how do you use the corresponding class selector in CSS?
• Various HTML specifications impose various restrictions on class names, but in browser practice, and according to HTML5, there are no limitations, except that a class name cannot contain whitespace characters. So class=000000-8 is valid.
• By CSS syntax rules, a CSS identifier cannot being with an unescaped digit. Therefore, a selector like .000000-8 is invalid. But the digit can be escaped, by CSS escaping rules(http://www.w3.org/TR/css3-syntax/#escaping) the selector
• .\30 00000-8or equivalently,.\00003000000-8 is valid and matches an element with class=000000-8.

SOLUTION 2:

• Basically, a name must begin with an underscore (_), a hyphen (-), or a letter(a–z), followed by any number of hyphens, underscores, letters, or numbers.
• There is a catch: if the first character is a hyphen, the second character must be a letter or underscore, and the name must be at least 2 characters long.
• -?[_a-zA-Z]+[_a-zA-Z0-9-]*
• Identifiers beginning with a hyphen or underscore are typically reserved for browser-specific extensions, as in -moz-opacity

SOLUTION 3:

• The article explains how to escape any character in CSS (and JavaScript),If you were to give an element an ID value of ~!@$%^&*()_+-=,./’;:”?><[]{}|`#, the selector would look like this:

    CSS:

<style>
  #\~\!\@\$\%\^\&\*\(\)\_\+-\=\,\.\/\'\;\:\"\?\>\<\[\]\\\{\}\|\`\#
  {
    background: blue;
  }
</style>
JavaScript:
<script>
 	 // document.getElementById or similar
  document.getElementById('~!@$%^&*()_+-=,./\';:"?><[]\\{}|`#');
  	// document.querySelector or similar
  $('#\\~\\!\\@\\$\\%\\^\\&\\*\\(\\)\\_\\+-\\=\\,\\.\\/\\\'\\;\\:\\"\\?\\>\\<\\[\\]\\\\\\{\\}\\|\\`\\#');
</script>

SOLUTION 4:

The complete regular expression is:

• -?(?:[_a-z]|[\200-\377]|\\[0-9a-f]{1,6}(\r\n|[ \t\r\n\f])?|\\[^\r\n\f0-9a-f])(?:[_a-z0-9-]|[\200-\377]|\\[0-9a-f]{1,6}(\r\n|[ \t\r\n\f])?|\\[^\r\n\f0-9a-f])*
• “-” and “_” are not allowed in your listed character if used directly. But you can encode them using a backslash foo\~bar or using the unicode notation foo\7E bar.

SOLUTION 5:

Any character except NUL is allowed in CSS class names in CSS (If CSS contains NUL ,the result is undefined).

• The links to explanation(http://mathiasbynens.be/notes/css-escapes) and demos (http://mathiasbynens.be/demo/crazy-class)showing how to use these names. Written down in CSS code, a class name may need escaping, but that doesn’t change the class name. E.g. an unnecessarily over-escaped representation will look different from other representations of that name, but it still refers to the same class name.
• Most other (programming) languages don’t have that concept of escaping variable names (“identifiers”), so all representations of a variable have to look the same. This is not the case in CSS.
• Note that in HTML there is no way to include space characters (space, tab, line feed, form feed and carriage return)(http://www.w3.org/TR/html/infrastructure.html#space-character)  in a class name
• attribute(http://www.w3.org/TR/html/dom.html#classes) , because they already separate classes from each other.
  So, if you need to turn a random string into a CSS class name: look out NUL and space, and escape (accordingly for CSS or HTML).

SOLUTION 6:

• In CSS, identifiers (including element names, classes, and IDs in selectors) can contain only the characters [a-z0-9] and ISO 10646 characters U+00A1 and higher, plus the hyphen (-) and the underscore (_); they cannot start with a digit, or a hyphen followed by a digit.
• Identifiers can also contain escaped characters and any ISO 10646 character as a numeric code (see next item). For instance, the identifier “B&W?” may be written as “B\&W\?” or “B\26 W\3F”.
• In CSS, identifiers may begin with ‘-‘ (dash) or ‘_’ (underscore). Keywords and property names beginning with ‘-‘ or ‘_’ are reserved for vendor-specific extensions. Such vendor-specific extensions should have one of the following formats:
• ‘-‘ + vendor identifier + ‘-‘ + meaningful name
• ‘_’ + vendor identifier + ‘-‘ + meaningful name
• For example, if XYZ organization added a property to describe the color of the border on the East side of the display, they might call it -xyz-border-east-color.
• Other known examples:
• -moz-box-sizing
• -moz-border-radius
• -wap-accesskey
• An initial dash or underscore is guaranteed never to be used in a property or keyword by any current or future level of CSS.
• Thus typical CSS implementations may not recognize such properties and may ignore them according to the rules for handling parsing errors.
• CSS 2.1 implementers should always be able to use a CSS-conforming parser, whether or not they support any vendor-specific extensions

SOLUTION 7:

CSS Specification:

• In CSS, identifiers (including element names, classes, and IDs in selectors) can contain only the characters [a-zA-Z0-9] and ISO 10646 characters U+00A0 and higher, plus the hyphen (-) and the underscore (_); they cannot start with a digit, two hyphens, or a hyphen followed by a digit.
• Identifiers can also contain escaped characters and any ISO 10646 character as a numeric code.
• For instance, the identifier “B&W?” may be written as “B\&W\?” or “B\26 W\3F”.

SOLUTION 8:

• Since HTML5 it’s been valid to start an HTML element ID with a number. For example

[id="10"] {color: red;}

However, another possibility is to use a unicode character instead of a number. For example, instead of the attribute selector, you can do this:

#\31 0 {color: red;}

• The same number and selector rules apply with class names.
• You can write a class starting with a number in HTML but to get the selector to work in CSS you either need to use the attribute selector or escape it with unicode.
• To show the prior example with a class instead:

<div class="10">hello</div>

You can either do this:

[class="10"] {color: red;}

Or this:

.\31 0 {color: red;}

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