Why it matters: It impacts readability and comprehension. A clear visual distinction between headings, subheadings and paragraphs reduces users' mental overload and helps them digest the content. It also brings visual benefits — your page will look better and feel lighter.
How to test it: While reviewing the content inside your product, make sure that headings, subheadings and paragraphs are of different font sizes and weights. Check this example from Slack: