Detailed introduction to the class name problem in css

The following CSS class names starting with numbers will not take effect:

.1st{ color: red; }

A legal CSS class name must start with one of the following:

1. Underscore_

2. Dash-

3. Letter a - z

followed by other _, - numbers or letters.

(Recommended tutorial:CSS Getting Started Tutorial)

Using regular expressions, a legal CSS class name:

-?[_a-zA-Z]+[_a-zA-Z0-9-]*

According to CSS According to the description in the standard, if the class name starts with a dash -, the second character must be an underscore _ or a letter, but actual testing found that in addition to the two mentioned, it is also valid to be followed by another dash.

The following is the test code and results:

should apply red color
 
should apply red color
 
should apply red color
 
should apply red color
 
should apply red color
 
should apply red color
 .1st { color: red; } .-1foo { color: red; } .-_foo { color: red; } .--foo { color: red; } .-foo { color: red; } .foo { color: red; }

The results are as shown:

## Recommended related video tutorials:

css video tutorial

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