What:

This pull request resolves issue #4267 by fixing @starting-style at-rule nesting.

Why:

Users of Less.js may wish to use newer features of CSS supported by newer versions of most browsers. Estimated support for @starting-style is 86.48%+ (https://caniuse.com/mdn-css_at-rules_starting-style).

Checklist:

• Documentation
• Added/updated unit tests
• Code complete

Per comment on #4289 I worked to add support for the starting-style at-rule by bolstering base AtRule tree node capabilities.

The following Less:

[popover]:popover-open {
    opacity: 1;
    transform: scaleX(1);

    @starting-style {
        opacity: 0;
        transform: scaleX(0);
    }
}

becomes:

[popover]:popover-open {
  opacity: 1;
  transform: scaleX(1);
  @starting-style {
    opacity: 0;
    transform: scaleX(0);
  }
}