A few rules every screen follows:
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Body text is at least 17 points. In the app it grows with your phone’s text size; on the website you can zoom in as far as you need.
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Buttons are at least 48 pixels tall, so they are easy to tap.
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Color never carries a meaning alone: a reading’s status always has an icon and a word (Good, Check, Act now).
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A screen reader hears each history chart in words: the latest reading and how many tests were in the good range.
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There is a light and a dark look.