CSS features undergo significant discussions before being defined by the W3 Consortium and gradually implemented in browsers, with the guide focusing on five testable features: CSS subgrid, Flexbox gaps, content-visibility, contain-intrinsic-size, and the :is and :where pseudo-classes. CSS subgrid enhances grid layout capabilities by allowing grandchildren to adopt a grid's tracks, supported mainly by Firefox 71+ and soon by Chromium-based browsers. Flexbox gaps solve spacing issues between flex items, supported widely except by Safari and Internet Explorer. The content-visibility property, supported by Chrome 85+, Edge 85+, and Opera 71+, improves rendering performance by managing off-screen elements. The contain-intrinsic-size property prevents collapsibility by setting explicit dimensions for elements, supported by Chrome 83+, Edge 83+, and Opera 69+. The :is and :where pseudo-classes simplify CSS selector lists with different specificity levels, where :is is more widely supported. Developers are encouraged to use these features cautiously, considering browser support, while monitoring ongoing CSS developments to facilitate more efficient frontend development.