Landmarks help assistive-technology users navigate to and between areas of a page, and they improve the efficiency of in-page navigation.
Landmarks are to sections what continents are to countries—they help break the interface up into a few large, semantically distinct areas such as headers, footers and navigation blocks. The main landmark defines the unique content of the page: the most likely reason a user visited the page in the first place.
The heading levels (1-6) are often considered a way of describing and determining the “importance” of a heading, with h1 being the most important. This might be reflected in the visual appearance of headings—higher-level headings typically appear as bigger and bolder text than lower-level headings.
But you can't just put an “important” heading anywhere. Its heading level relates to how much content it refers to, and that has to do with structure.
Reflecting structure accurately allows screen-reader users to use their screen reader's heading-navigation feature to navigate by heading level, building up an idea of document structure. It also ensures that, for people who use custom stylesheets, heading structure is preserved even when the appearance of headings is changed.... Read more about ✎ Technique: Heading structure
For a readable interface, it's important to use visual cues to group or separate content to show which content relates to other content. This makes content easier to read, especially for people who have difficulty reading on-screen content and people who use screen readers.
When grouping content visually, you must make sure that you're not skewing the source order and breaking up the same content for screen-reader users.
When positioning elements using CSS, it's possible to place them in a position that does not reflect their location in the source order. This can have unexpected side effects for keyboard-navigation users.
Icon fonts are a popular and effective method for providing scalable symbols that can be used to label controls and provide graphical information. The information provided by icons also needs to be available to people who can’t see them.
The most appropriate alternative text for an image depends very much on the context of the image in question. You must provide information for that image that takes into account its purpose and also the surrounding text on the page. The same image might need different alternative text depending on how it's used.... Read more about ✎ Technique: Writing alternative text
Each image requires alternative text that is equivalent to the information it conveys, unless the image is purely decorative and provides no useful information. So you need to be careful about what value you give the alt attribute.