Built-in Accessibility Features:
IMAGES: Use alt tags when inserting images.
When you hover your mouse over an image on a website, the little words that pop up are called alt tags. Never copy and paste an image into the content area, always use the Image Manager icon 
to upload an image. You will then be required to apply Alt Text to the image before inserting it.
TABLES: Use a simple table structure and identify the header row.
If you must use a table to present your data, it is recommended to indicate how many heading rows or heading columns are in the table. You can also add a caption and a summary, which can be used to improve the accessibility of the table. Also, ensure that tables don't contain split cells, merged cells or nested tables.
Use the Insert Table icon to access the Table Wizard, then click on the Accessibility tab:
Best Practices:
Describe the hyperlinks.
When creating a hyperlink, describe the link rather than just telling the reader to "click here". For example: "To learn more about immunizing your child, check out Ottawa Public Health."
Do not underline text.
Only hyperlinks should be underlined; they are underlined automatically.
Put periods in abbreviations.
A screen reader won't recognize the abbreviation without periods, and will instead read it out phonetically as a word.
Avoid Images of Text.
If you must use an image with text in it, repeat that text in the document. In alt text, briefly describe the image and mention the existence of the text and its intent.
Design for Visual Disabilities such as blindness, Low-Vision and color-blindness.
If you must use an image with text in it, repeat that text in the document. In alt text, briefly describe the image and mention the existence of the text and its intent.
Use colour control and do not use bright flashing images.
Black text on a white background is best because it's readable for most audiences. Bright, strobing images or media and the colour red can cause photo epileptic seizures. Seizure-inducing media is becoming more common in web video, especially HD-quality video that includes strobing special effects. Such media must be avoided!
Read more about accessible colour control and images here.
Keep your information simple.
If you're putting a lot of text onto your website, break it into smaller paragraphs and use simple, straightforward language.
Reduce the use of PDFs where possible, or make your PDFs accessible.
The majority of PDF files on the web were probably created in Microsoft Word. The good news is that it is possible to create accessible PDF files in Office, as long as the following requirements are met:
- The file must be accessible. That includes providing alternative text for images, proper headings, appropriate link text, etc. For more information, read this tutorial on Microsoft Word.
- Office 2000-2003 users must have Acrobat installed, as well as the add-in. Office 2007 users must have either Acrobat or the Microsoft PDF add-in installed. Office 2010 users can create tagged PDF files natively or with the Adobe add-in.
- The file must be exported correctly. If a file is created by printing to PDF, it will not be correctly tagged.
For instructions on how to save a Word document as a PDF, select the version of MS Word that you are using:
How to make PDF documents accessible (WebAIM website).