An introduction to the class name issue 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:

<p class="1st">should apply red color</p>
<p class="-1foo">should apply red color</p>
<p class="-_foo">should apply red color</p>
<p class="--foo">should apply red color</p>
<p class="-foo">should apply red color</p>
<p class="foo">should apply red color</p>
.1st {
  color: red;
}
.-1foo {
  color: red;
}
.-_foo {
  color: red;
}

.--foo {
  color: red;
}

.-foo {
  color: red;
}
.foo {
  color: red;
}

The results are as shown:

Recommended video tutorial: css Video tutorial

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