The lang attribute specifies the primary language used in contents and attributes containing text content of particular elements.

A literal character specifies exactly itself to be matched in the input text.

The keyPoints attribute indicates the simple duration of an animation.

The if...else statement executes a statement if a specified condition is truthy. If the condition is falsy, another statement in the optional else clause will be executed.

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.