The starting point for the observation is always the continuous text, which is what we write into the p-tags on the web. This text is the index, meaning it keeps the font size of 100 percent. All other elements are derived from that.
According to this, the first step is getting the text to a comfortably legible size. A lot of this is just a matter of taste. For me, this value is at 18 pixels, I don’t want it to be any lower. Common publications are mostly around 14 pixels. No matter which size you define, it is important that it is the foundation for all other elements.
Headlines (hl) should have 180 to 200 percent of the original size, secondary headings (h2) 130 to 150 percent, and tertiary headings (h3) should only be slightly bigger than the running text. In many cases, you’ll see h3 as a simple bold text. You should be able to go up to 125 percent without problems. Footnotes shouldn’t exceed 75 percent of the size of the continuous text.
This is good advice and reading the entire article is worth your time if you use words. This is also a good time to recommend The Elements of Typographic Style by Robert Bringhurst.