Use responsive design to allow your content to zoom and respond to various screen sizes.
Present content without loss of information or functionality, and without requiring scrolling in two dimensions, except for parts of the content which require two-dimensional layout for usage or meaning.
Examples of content which require two-dimensional layout are images, maps, diagrams, video, games, presentations, data tables, and interfaces where it is necessary to keep toolbars in view while manipulating content.
Anyone who needs to change the font or text display properties to read the content will benefit from this guideline. This includes people with low vision who use a bigger font, and people with dyslexia, reading, or other cognitive disabilities, who may have specific spacing requirements.
Paragraphs of text are a fundamental core of web content, so it's important to display them in a fashion that is optimally readable to the majority of your audience without requiring them to change their display settings. For people who do need to customize display of text to make it easier to read, it's important to support this customization rather than forcing them to read the text the way you specify.
For the most part, this means applying typesetting best practices, which predate the web.
Some people with reading difficulties or visual impairments need to customize the display of text to make it easier to read. When text is presented as an image of text, that limits their ability to change the appearance of that text. So wherever possible, use text along with CSS to apply styling (such as color, typeface, or size).
If you use an online content editor to write content, the styling will happen automatically. If you feel that you need text that deviates from the style, formatting options...
Some people with reduced visual acuity may not need screen magnification software or screen readers but might still have problems reading smaller fonts. So the ability to allow easy enlargement of text size is important.
There are a number of ways to zoom web-page content in to enlarge text without needing to provide explicit controls to do so. To avoid diminishing this variety of user control, you have to be careful how you code font sizes.