Custom Elements allow you to build Typography settings for any element on your website.
Custom Elements can be created under Google Fonts → Custom Elements.
The ‘Selectors’ field accepts any valid CSS Selectors. That means there is no element on your website that is off-limits.
A Selector can target a single element, or multiple elements separated by commas. For example:
.widget-title, #form, table, h1
Once you have created your Custom Elements. Press ‘Save Changes’. You will then be able to customize your Elements under Google Fonts → Advanced Settings → Custom Elements.
Further Reading: How to Find CSS Selectors