A lookbehind assertion "looks behind": it attempts to match the previous input with the given pattern, but it does not consume any of the input — if the match is successful, the current position in the input stays the same. It matches each atom in its pattern in the reverse order.

The k3 attribute defines one of the values to be used within the arithmetic operation of the <feComposite> filter primitive.

A unicode character class escape is a kind of character class escape that matches a set of characters specified by a Unicode property. It's only supported in Unicode-aware mode. When the v flag is enabled, it can also be used to match finite-length strings.

The do...while statement creates a loop that executes a specified statement as long as the test condition evaluates to true. The condition is evaluated after executing the statement, resulting in the specified statement executing at least once.

The dur attribute indicates the simple duration of an animation.

The end attribute defines an end value for the animation that can constrain the active duration.

A block statement is used to group zero or more statements. The block is delimited by a pair of braces ("curly braces") and contains a list of zero or more statements and declarations.

The opacity attribute specifies the transparency of an object or of a group of objects, that is, the degree to which the background behind the element is overlaid.

The cy attribute define the y-axis coordinate of a center point.

Classes are a template for creating objects. They encapsulate data with code to work on that data. Classes in JS are built on prototypes but also have some syntax and semantics that are unique to classes.