The CSS Working Group has published an updated Working Draft of Containment Level 3. Containment indicates that an element’s subtree is independent of the rest of the page. This enables heavy optimizations by user agents. Level 3 defines inline-size containment, along with the terms, properties, units, and at-rule needed for Container Queries.
This update represents 8 months of refining the functionality and syntax based on feedback from both authors and implementors. For example:
• Queries are automatically resolved against a container that has the appropriate container-type, without additional syntax.
• Size queries are streamlined, and more similar to Media Queries, by removing the function-based syntax.
• Block-size container-type was removed, as it was not viable for implementors, and has few use-cases.
• In order for style-queries to work with inherited styles, all elements are style containers by default. That also removes the need for a container-type of style.
• General-enclosed is allowed in the query syntax, for the sake of forward compatibility.
Please send feedback by either filing an issue in GitHub (preferable) or sending mail to the (archived) public mailing list www-style@w3.org with the spec code ([css-contain-3]) and your comment topic in the subject line. (Alternatively, you can email one of the editors and ask them to forward your comment.)