At default browser level all text must have a minimum calculated size of 11px and all core content must have a minimum calculated size of 13px.

Rationale

Having a minimum text size will reduce the number of users who need to make use of browser based text resize or page zoom. This is a particular issue with an ageing audience, many of whom will not consider themselves as having low vision and there will not have access to assistive technology or be familiar with browser tools to resize content.

Definitions

Core content

The content that is required to fulfil the core purpose of the document or feature.

Techniques

Pass:

<style>
    body {
        font-size: 62.5%; /* Set default size of 1em to 10px */
    }

    .news-article p {
        font-size: 1.3em; /* Primary content size set to 13px */
    }

    .news-supplimentary-links {
        font-size: 1.2em; /* Secondary content size set to 12px */
    }
</style>

Fail:

<style>
    body {
        font-size: 62.5%; /* Set default size of 1em to 10px */
    }

    .news-article p {
        font-size: 1.2em; /* Primary content size set to 12px */
    }

    .news-supplimentary-links {
        font-size: 1em; /* Secondary content size set to 10px */
    }
</style>

Test