The :valid CSS pseudo-class represents any <input> or <form> element whose content validates correctly according to the input's type setting. This allows to easily make valid fields adopt an appearance that helps the user confirm that their data is formatted properly.
The <angle>CSS data type represents angle values. Positive angles represent clockwise angles, negative angles represent counterclockwise angles. Its syntax is a <number> data type immediately followed by the unit (deg, grad, rad or turn). Like for any CSS dimension, there is no space between the unit literal and the number.
The contain property allows an author to indicate that an element and its contents are, as much as possible, independent of the rest of the document tree. This allows the browser to recalculate layout, style, paint, size, or any combination of them for a limited area of the DOM and not the entire page. This property is useful on pages that contain a lot of widgets that are all independent as it can be used to prevent one widget's CSS rules from changing other things on the page.
The background-size CSS property makes it possible to adjust the size of background images, instead of the default behavior of tiling the image at its full size. You can scale the image upward or downward as desired.
Image sprites are used in numerous web apps where multiple images are used. Rather than include each image as a separate image file, it is much more memory and bandwidth-friendly to send them as a single image, so the number of HTTP requests is reduced.
CSS gradients are new types of <image> added in the CSS3 Image Module. Using CSS gradients lets you display smooth transitions between two or more specified colors. This lets you avoid using images for these effects, thereby reducing download time and bandwidth usage. In addition, because the gradient is generated by the browser, objects with gradients look better when zoomed, and you can adjust your layout much more flexibly.
Usually HTML pages can be considered two-dimensional, because text, images and other elements are arranged on the page without overlapping. There is a single rendering flow, and all elements are aware of the space taken by others. The z-index attribute lets you adjust the order of the layering of objects when rendering content.
Stacking context is the three-dimensional conceptualization of HTML elements along an imaginary z-axis relative to the user who is assumed to be facing the viewport or the webpage. HTML elements occupy this space in priority order based on element attributes.
The <gradient>CSS data type denotes a CSS <image> made of a progressive transition between two or more colors. A CSS gradient is not a CSS <color> but an image with no intrinsic dimensions; that is, it has no natural or preferred size, nor a preferred ratio. Its concrete size will match the one of the element it applies to.