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sibling-count() and sibling-index(): The sibling-count() and sibling-index() CSS functions return integers that are useful to style elements based on their positions among siblings or on the number of siblings, for example as part of a calc() expression.
Intl.Locale info: The getter methods of the Intl.Locale API provide supplemental information about a Unicode locale, such as the locale's calendar (for example, the first day in a week or the weekend start day), writing direction, 12- or 24-hour cycles, and numbering system.
field-sizing: The field-sizing CSS property allows form controls such as <textarea> to be sized based on their content.
Container style queries: Container style queries with the @container at-rule apply styles to an element based on the values of custom properties of its container.
:open: The :open CSS pseudo-class matches elements that have open states, like <details>, <dialog>, or <select>, based on their state.

Declarative shadow DOM: The shadowrootmode attribute on <template> creates a shadow root without the use of JavaScript. It is a declarative alternative to the attachShadow() method.
Array.fromAsync(): The Array.fromAsync() static method copies items from an async iterable object to make a new array.
AVIF: AVIF (AV1 Image File Format) is an image format based on the AV1 video format.
:has(): The :has() CSS functional pseudo-class matches an element if any of the selectors passed as parameters would match at least one element.
Lazy-loading images and iframes: The loading="lazy" attribute for <img> and <iframe> elements blocks loading the external resource until the user scrolls to that element's part of the page.