Based on my comment to the accepted answer, there are a lot potential pitfalls that you may encounter by declaring font-sizes smaller than 12px. By declaring styles that lead to computed font-sizes of less than 12px, like so:
html {
font-size: 8px;
}
p {
font-size: 1.4rem;
}
// Computed p size: 11px.
You'll run into issues with browsers, like Chrome with a Chinese language pack that automatically renders any font sizes computed under 12px as 12px. So, the following is true:
h6 {
font-size: 12px;
}
p {
font-size: 8px;
}
// Both render at 12px in Chrome with a Chinese language pack.
// How unpleasant of a surprise.
I would also argue that for accessibility reasons, you generally shouldn't use sizes under 12px. You might be able to make a case for captions and the like, but again--prepare to be surprised under some browser setups, and prepared to make your grandma squint when she's trying to read your content.
I would instead, opt for something like this:
h1 {
font-size: 2.5rem;
}
h2 {
font-size: 2.25rem;
}
h3 {
font-size: 2rem;
}
h4 {
font-size: 1.75rem;
}
h5 {
font-size: 1.5rem;
}
h6 {
font-size: 1.25rem;
}
p {
font-size: 1rem;
}
@media (max-width: 480px) {
html {
font-size: 12px;
}
}
@media (min-width: 480px) {
html {
font-size: 13px;
}
}
@media (min-width: 768px) {
html {
font-size: 14px;
}
}
@media (min-width: 992px) {
html {
font-size: 15px;
}
}
@media (min-width: 1200px) {
html {
font-size: 16px;
}
}
You'll find that tons of sites that have to focus on accessibility use rather large font sizes, even for p elements.
As a side note, setting margin-bottom equal to the font-size usually also tends to be attractive, i.e.:
h1 {
font-size: 2.5rem;
margin-bottom: 2.5rem;
}
Good luck.