Readability describes the ease (or difficulty) with which content can be read. For web writing, readability should be at about the 8th grade level.
When writing a web page:
Keep in mind the kinds of things that will slow your reader down.
Filter them out or use, as appropriate.
If you're unsure of the level of your content, you can test it with Microsoft Outlook or Microsoft Word or an online tool. This blog post from medium.com (Tips and Tools for Improving Your Content Readability Score) provides more details of the benefits of readability and suggests available online tools.
Things that Impede Readability
Incorrect/lack of punctuation
Use the items in this list with discretion. A few explanatory notes:
ACS is a scientific society. Scientific terminology, technical terms, etc. are appropriate and necessary on some pages.
Italics and all-caps can be effective to emphasize keywords. When used excessively, they make it harder for users to focus on your message.
Make an effort to avoid underlining in your text. Users expect underlined content to indicate hyperlinks and to be clickable.
Images can also be effective to help users process content.