CSS におけるクラス名の問題の概要

.1st{
    color: red;
}

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

<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;
}