CSS属性值中的！default是什么意思？

You can find the following exact definition and a decent explanation in sass-lang website in its doc section (variable) - default value:

Normally when you assign a value to a variable, if that variable already had a value, its old value is overwritten. But if you’re writing a Sass library, you might want to allow your users to configure your library’s variables before you use them to generate CSS. To make this possible, Sass provides the !default flag. This assigns a value to a variable only if that variable isn’t defined or its value is null. Otherwise, the existing value will be used.

据我所知，Twitter Bootstrap使用LESS。另一方面，!default它实际上是Sass的一部分，用于为Sass变量（$var）提供默认值，这将使其在给定的上下文中无效，即使在Sass中也是如此。

此外，我!default在LESS文档中找不到任何引用，据我所知，它仅属于Sass。您确定在Bootstrap的源代码中找到了这个吗？因为老实说，我不记得在Bootstrap的样式表中看到Sass / SCSS代码。

就其价值而言，!CSS中唯一以开头的有效令牌是!important，您可能已经知道了。