The append() method of the StylePropertyMap interface adds the passed CSS value to the StylePropertyMap with the given property.

property

An identifier indicating the stylistic feature (e.g. font, width, background color) to add.

value

The value the given property should have.

None (undefined).

This example shows an extra background image value being added to the background-image property of the element, using HTMLElement.attributeStyleMap.

// get the button element
const buttonEl = document.querySelector("button");

// append another value to the background-image property set on the attribute
buttonEl.attributeStyleMap.append(
  "background-image",
  "linear-gradient(180deg, blue, black)",
);