Errors, Warnings, and Tips: What Do Each Mean?

Source: Using the Accessibility Checker by Microsoft.

When a user runs the Accessibility Checker, it presents accessibility errors, warnings, and tips. The following tables itemize the Accessibility Checker rules, what they check for, where to learn how to fix each issue, and why you should fix each one.

Errors

If content in the file makes it very difficult or impossible for someone with a disability to use, the Accessibility Checker classifies it as an error.

Errors
Rule	Accessibility Checker verifies	Why fix this?
All non-text content has alternative text (alt text).	All objects have alt text and the text doesn’t contain images or file extensions.	Screen readers speak the alternative text to describe images and other non-text content that users can’t see. Based on the alt text of non-text content, users should understand the purpose and meaning.
Tables specify column header information.	Tables and/or blocks of cells have the header box selected or a header row indicated.	Users rely on the table headings to understand the content that is subsequently read by the screen reader. Also, assistive technology often uses the table header row to help convey to the user the current cursor location in the table and to provide information that enables the user to navigate the table.
Documents use heading styles	Content is organized with headings and/or a Table of Contents (TOC).	Headings and TOCs provide structural context to users and enable navigation and easier searching in the document.

Warnings

If the content in most (but not necessarily all) cases is difficult for people with disabilities to understand, the Accessibility Checker gives a warning.

Warnings
Rule	Accessibility Checker verifies	Why fix this?
Hyperlink text is meaningful.	Link text makes sense as standalone information, providing accurate information about the destination target.	Based on the text, users decide whether to click a hyperlink. The text should provide clear information about the link destination.
Table has a simple structure.	Tables are simple rectangles with no split cells, merged cells, or nesting.	Users navigate tables via keyboard shortcuts and assistive technology, which rely on simple table structures.
Tables don’t use blank cells for formatting.	There are no entirely blank rows or columns in the table.	Blank table cells can mislead a user into thinking that there is no more content in the table.
Avoid the use of repeated blank characters.	There are no runs of blank spaces, tabs, or carriage returns.	Spaces, tabs, and empty paragraphs often are read as blanks by assistive technology. After hearing several “blanks,” people might think that they have reached the end of the information.
Avoid the use of text color that doesn't stand out from the background color.	The contrast between text and background colors is sufficient.	People with low vision often find it hard to read text that does not contrast with the background.

Tips

When there is content that people with disabilities can understand but that could be better organized or could be presented in a way that can improve their experience, you see a tip.

Tips
Rule	Accessibility Checker verifies	Why fix this?
Closed captions are included for inserted audio and video.	All audio and video objects have closed captioning.	Without captioning, the information in a video or audio segment may be entirely lost to people with disabilities.
No image watermarks are used.	There are no watermarks.	Watermarks might be misunderstood as being part of the main content on the page and could cause confusion.
All headings are in the correct order.	All headings follow a logical order.	Sequential headings with appropriate levels help users navigate, search, and understand the document’s organization.