CSS

Content does not restrict its view and operation to a single display orientation, such as portrait or landscape, unless a specific display orientation is essential (i.e., bank check). 

Make it easier for people to complete input fields requesting personal information.

Use responsive design to allow your content to zoom and respond to various screen sizes.

The contrast of icons and graphical objects is just as important as that of text. Use sufficient contrast for all elements on the screen, with a minimum contrast ratio of 3:1.

People with a variety of print disabilities (including low vision, dyslexia, other cognitive disabilities) can benefit from being able to increase the spacing between lines, words, and/or letters on their own device.

Color-contrast-checking tools can compare two colors and report on the contrast ratio between them. Some tools will allow you to adjust these values until the ratio is sufficient, helping you to choose a color scheme that avoids contrast problems.

Avoid referring to a button, menu, or other item in the page only by its position on the page; instead, use additional information that describes the content.

Consistency is one of the cornerstones of good usability.

In long lists of links, it's helpful to show users which links they've already followed so they can focus on unseen content. Browsers do not tend to let you style visited links with anything but the color property.

Paragraphs of text are a fundamental core of web content. It's important to display them in a fashion that is optimally readable to the majority of your audience.