Cascade layers in CSS provide a way to have more control over the specificity and order in which styles are applied. To create a layer, use the @layer rule, enclosing all the styles for that particular layer within its curly braces. The precedence goes from top-to-bottom where the last defined layer has the highest specificity.

@layer base {
  a {
    color: blue;
  }
}

/* This layer takes precedence over the base layer */
@layer content {
  a {
    color: red;
  }
}

It's also possible to define the order at the top:

@layer base, content;

/* This layer takes precedence over the base layer */
/* Even though it is used before the base layer */
@layer content {
  a {
    color: red;
  }
}

@layer base {
  a {
    color: blue;
  }
}

Styles that are not inside a @layer rule, will always take precedence over the styles within layers. Regardless of the order. So when using layers it's probably a good idea to put all the styles inside a layer.

@layer base, content;

@layer base {
  a {
    color: blue;
  }
}

/* The links will be green, */
/* because this takes precedence over the base and content layers. */
a {
  color: green;
}

@layer content {
  a {
    color: red;
  }
}

!important

Unfortunately the layers do not take precedence over a style that has been set with !important. It's also not possible to override a style with !important, by using !important in a layer that has higher specificity. When using cascading layers, it's really best not to use !important at all!