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When you're designing for mobile, it's important to remember that some people will need to change the size of the content to read it easily.
There are so many ways to view content on mobile devices, from zooming in to orienting the screen differently, so a failure to design in a dynamic environment can lead to accessibility issues. There are different ways you can attend to this, whether you're writing an app for iOS or Android.
Considerations for iOS
If you manually limit how large your text scales it may cause issues for people who need a bigger font. You want the text to scale dynamically instead of stopping at the point you set, because that size may still not be visible to all people.
Minimum font-size for body text should be 17pt.
Considerations for Android
You can use a tool called the Android Emulator to preview how your app works in the context of different android devices. This can help you stay aware of how people who use different types of androids experience your app.
Again, users can only zoom in on specified areas, so you'll need to use scale-independent pixels to determine the size of your text.
There isn't a tool like Dynamic Type in Android, so you'll need to scale the text at different sizes yourself. Material design has a resource that makes a good starting point.
Minimum font-size for body text should be 16sp:
You'll also want to remember to make your content container scrollable. If it's not, you'll just cut off the extra content when a person zooms in.
Also consider that apps may contain HTML views. There are a couple things to keep in mind here for accessibility as well.
In your HTML, make sure to use relative units for responsive design. Here's a resource from The CSS Layout Workshop for applying them to your HTML via CSS. If you don't use relative units, people could end up having to use horizontal and vertical scrolling to view content, which violates WCAG 1.4.10.
Note that a person's pinch-to-zoom capability will be frustrated on mobile < iOS10 if you add
maximum-scale=1.0 to the
meta element. This could force a person to view your content without zooming, which could make that content unviewable to them.