best practice font size for mobile - html

Question

I have seen this question on SO:
What are the most common font-sizes for H1-H6 tags
with this being the recommended font sizes fo H tags:
h1 { font-size: 2em; }
h2 { font-size: 1.5em; }
h3 { font-size: 1.17em; }
h5 { font-size: .83em; }
h6 { font-size: .75em; }
Is there a 'best practice' for these for mobile phones? -say iphone screen size?

Answer 1

The font sizes in your question are an example of what ratio each header should be in comparison to each other, rather than what size they should be themselves (in pixels).
So in response to your question "Is there a 'best practice' for these for mobile phones? - say iphone screen size?", yes there probably is - but you might find what someone says is "best practice" does not work for your layout.
However, to help get you on the right track, this article about building responsive layouts provides a good example of how to calculate the base font-size in pixels in relation to device screen sizes.
The suggested font-sizes for screen resolutions suggested from that article are as follows:
#media (min-width: 858px) {
html {
font-size: 12px;
}
}
#media (min-width: 780px) {
html {
font-size: 11px;
}
}
#media (min-width: 702px) {
html {
font-size: 10px;
}
}
#media (min-width: 624px) {
html {
font-size: 9px;
}
}
#media (max-width: 623px) {
html {
font-size: 8px;
}
}

Answer 2

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.

Answer 3

The whole thing to em is, that the size is relative to the base. So I would say you could keep the font sizes by altering the base.
Example: If you base is 16px, and p is .75em (which is 12px) you would have to raise the base to about 20px. In this case p would then equal about 15px which is the minimum I personally require for mobile phones.