What are the principles of accessible design

Provide appropriate alternative text:
Alternative text provides a textual alternative to non-text content in web pages. It is especially helpful for people who are blind and rely on a screen reader to have the content of the website read to them.

Provide appropriate document structure
Headings, lists, and other structural elements provide meaning and structure to web pages. They can also facilitate keyboard navigation within the page.

Provide headers for data tables
Tables are used online for layout and to organize data. Tables that are used to organize tabular data should have appropriate table headers (the element). Data cells should be associated with their appropriate headers, making it easier for screen reader users to navigate and understand the data table.


Ensure users can complete and submit all forms
Ensure that every form element (text field, checkbox, dropdown list, etc.) has a label and make sure that label is associated to the correct form element using the element. Also make sure the user can submit the form and recover from any errors, such as the failure to fill in all required fields.

Ensure links make sense out of context
Every link should make sense if the link text is read by itself. Screen reader users may choose to read only the links on a web page. Certain phrases like "click here" and "more" must be avoided.

Caption and/or provide transcripts for media
Videos and live audio must have captions and a transcript. With archived audio, a transcription may be sufficient.

Ensure accessibility of non-HTML content, including PDF files, Microsoft Word documents, PowerPoint presentations and Adobe Flash content.
In addition to all of the other principles listed here, PDF documents and other non-HTML content must be as accessible as possible. If you cannot make it accessible, consider using HTML instead or, at the very least, provide an accessible alternative. PDF documents should also include a series of tags to make it more accessible. A tagged PDF file looks the same, but it is almost always more accessible to a person using a screen reader.

Allow users to skip repetitive elements on the page
You should provide a method that allows users to skip navigation or other elements that repeat on every page. This is usually accomplished by providing a "Skip to Main Content," or "Skip Navigation" link at the top of the page which jumps to the main content of the page.

Do not rely on color alone to convey meaning
The use of color can enhance comprehension, but do not use color alone to convey information. That information may not be available to a person who is colorblind and will be unavailable to screen reader users.

Make sure content is clearly written and easy to read
There are many ways to make your content easier to understand. Write clearly, use clear fonts, and use headings and lists appropriately.

Make JavaScript accessible
Ensure that JavaScript event handlers are device independent (e.g., they do not require the use of a mouse) and make sure that your page does not rely on JavaScript to function.


References:
https://webaim.org/intro/