Get the default value of a CSS property

const getDefaultProperty = function (tagName, property) {
    // Create new element
    const ele = document.createElement(tagName);

    // Append to the body
    document.body.appendChild(ele);

    // Get the styles of new element
    const styles = window.getComputedStyle(ele);

    // Get the value of property
    const value = styles.getPropertyValue(property);

    // Remove the element
    document.body.removeChild(ele);

    // Return the value of property
    return value;
};

getDefaultProperty('div', 'font-size');

// Or
getDefaultProperty('div', 'fontSize');