The CSSStyleDeclaration.getPropertyPriority() method interface returns a string that provides all explicitly set priorities on the CSS property.

property

A string representing the property name to be checked.

A string that represents the priority (e.g. "important") if one exists. If none exists, returns the empty string.

The following JavaScript code checks whether margin is marked as important in a CSS selector rule:

const declaration = document.styleSheets[0].cssRules[0].style;
const isImportant = declaration.getPropertyPriority("margin") === "important";