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.
The <image>CSS data type represents a 2D image. There are two kinds of images in CSS: plain static images, often referenced using a URL, and dynamically-generated images like gradients or representations of parts of the tree.
The CSS linear-gradient() function creates an <image> which represents a linear gradient of colors. The result of this function is an object of the CSS <gradient> data type. Like any other gradient, a CSS linear gradient is not a CSS <color> but an image with no intrinsic dimensions; that is, it has neither natural or preferred size, nor ratio. Its concrete size will match the size of the element it applies to.
The <number>CSS data type represents a number, either integer or fractional. Its syntax extends the one of the <integer> data value. To represent a fractional value, add the fractional part — a '.' followed by one or several decimal digits — to any <integer> data value. Like for <integer> data type, there isn't any unit associated to a <number>, which is not a CSS dimension.
The CSS radial-gradient() function creates an <image> which represents a gradient of colors radiating from an origin, the center of the gradient. The result of this function is an object of the CSS <gradient> data type.
This works similarly to the standard radial gradients as described by radial-gradient(), but it automatically repeats the color stops infinitely in both directions, with their positions shifted by multiples of the difference between the last color stop's position and the first one's position.
The <time>CSS data type denotes time dimensions expressed in seconds or milliseconds. They consists of a <number> immediately followed by the unit. Like for any CSS dimension, there is no space between the unit literal and the number.
The <timing-function>CSS data type denotes a mathematical function that describes how fast one-dimensional values change during transitions or animations. This in essence lets you establish an acceleration curve, so that the speed of the animation can vary over its duration. These functions are often called easing functions.
The unset CSS keyword is the combination of the initial and inherit keywords. Like these two other CSS-wide keywords, it can be applied to any CSS property, including the CSS shorthand all. This keyword resets the property to its inherited value if it inherits from its parent or to its initial value if not. In other words, it behaves like the inherit keyword in the first case and like the initial keyword in the second case.
Each element in the top layer's stack has a ::backdroppseudo-element. This pseudo-element is a box rendered immediately below the element (and above the element below the element in the stack, if any), within the same top layer.
The :any-linkCSSpseudo-class represents an element that acts as the source anchor of a hyperlink independent of whether it has been visited, that is, it matches every <a>, <area> or <link> elements with an href attribute. So, it matches all elements that match :link or :visited.