Accessible text content boils down to understandability. Don't overcomplicate your content. Instead, aim for content that is easy to understand. A lot of these guidelines apply no matter where the content is.
For a web page, provide a short title that describes the page content. One that distinguishes it from other pages. The page title is often the same as the main heading of the page. Put the unique and most relevant information first. For example, put the name of the page before the name of the organization. Include the current step in the page title for pages that are part of a multi-step process.
The first heading on the page should be an h1 heading. Don't hide the h1 at the end of the page.
Heading levels should be logical, don't skip heading levels.
Headings and labels must describe the topic or purpose. Don't use headings for visual effects. Use CSS styles to create visual effects.
Use section headings to organize the content. It helps make the text much more understandable. You must have section headings on the AAA level but it's always a good practice.
Clear, simple, and concise language makes the most accessible text. You should always aim for plain language. Use inclusive language.
If there is something important in the content, you can bold it. But don't overdo it. But there are some things you should avoid.
Avoid cursive text. It makes the spaces between the letters and words smaller. For some people, it might become hard to understand where a word ends and another one begins.
Avoid capitalized text. Capitalized text is harder to read for everyone. So avoid capitalizing headings or links as a general rule. One capitalized word is easier to read than a whole sentence.
Don't underline text in a context where you might have links. The most common link style is an underline. People are likely to expect underlined text to be a link. And when the text doesn't take you anywhere when you click on it, it might be both confusing and frustrating.
Pay attention to link texts. Some screen reader users only check the links on the page. The clearer the link texts are, the better it is for everyone else as well. In general, you should avoid link texts such as "click here", it's better to provide more context. It is quite easy to fail even the Level A criterion, so I recommend paying attention to links.
Give users a warning if the link opens into another tab or window. For example, add "(opens a new window)" into the link text. In general, you should avoid opening links in a new window. At some point, there was a trend to open all external links in a new window. This is not necessary.
Use tables correctly, and provide the users with a table caption.
If you have headers in your table, use table headers. I recommend always using table headings. Otherwise, the content is not very understandable for assistive technologies.