CSS nesting relaxed syntax update

Earlier this year Chrome shipped CSS nesting in 112, and it's now in each major browser.

However, there was one strict and potentially unexpected requirement to the syntax, listed in the first article of the invalid nesting examples. This follow up article will cover what has changed in the spec, and from Chrome 120.

# Nesting element tag names

One of the most surprising limitations in the first release of CSS nesting syntax, was the inability to nest bare element tag names. This inability has been removed, making the following CSS nesting valid:

.card {
  h1 {
    /* this is now valid! */
  }
}

/* the same as */
.card {
  & h1 {
    /* this is now valid! */
  }
}

This becomes really sweet when nesting ordered, unordered or definition lists:

dl {
  dt {
    /* dl dt styles */
  }

  dd {
    /* dl dd styles */
  }
}

# What changed to allow this nesting?

It's largely thanks to some exploring and prototyping by Chrome engineers, combined with the desire from the community and CSS Working Group.

There was a decent amount of doubt that the CSS parser could be taught to differentiate between a tag name (div) and a property name (visibility) as the parser currently has no concept of looking ahead. To understand that the token is a property, the browser needs to look ahead and see if a : follows the unknown token. Therefore, in the original spec, the & symbol was used to indicate to the browser that what follows was nested, and not a regular CSS property and value.

Fortunately, an engineer discovered that when the parser failed to parse the nested selector as a property as per the normal consumption pass, it could be restarted in a mode that assumed a selector instead of a property. Parsing resumes, acknowledging the nesting as selector nesting. It's fast enough and reliable enough that it was determined sufficient enough to release the syntax.