Evaluate color combinations in terms of how their contrast affects readability. (Visitors quickly exit sites that strain their eyes.)
Never use Flash for a page header. It is not supported by Apple devices, usually increases the time it takes for a Web page to load, and its content cannot be indexed easily by search engines. In addition, its animation may distract visitors from concentrating on the content of the page.
Only use images as text-free backgrounds for formatted text that can be indexed by search engines (e.g., your organization’s name, slogan, contact information, etc.).
Ensure that your page header is viewable across all devices without horizontal scrolling and that it displays in its entirety in printouts.
Article(s) and Aside(s)
Present the Web page’s topic in the content of one or more semantic sectioning article elements.
Include semantic sectioning aside elements only when their content complements the Web page’s articles.
Never include an invisible, grayed-out, or nonsensical list of keywords. (Search engines penalize sites utilizing this trick.)
Use descriptive text labels for links to give them context and improve their Web page’s search engine optimization (SEO). In other words, do not use irrelevant labels such as "click here".
Code a link to an external Web site so that it opens the page in a new browser window. This will prevent visitors from permanently exiting your site.
Specify the size of a download in its link text.
Include one or more links to return to the top of a Web page – particularly for long pages when displayed on small devices.
Avoid the use of stock images that do not reflect the unique nature of your Web site. (Visitors may remember having seen these images in other sites.)
Use only Web-optimized images which will not significantly increase the load times of your site’s pages.
Use images that accommodate retina displays’ higher resolution.
Use descriptive alt texts (i.e., alternate texts) for images to accommodate slow Internet connections, text-only browsers, and screen readers for the visually impaired.
Use thumbnails to quickly load smaller versions of many images with links to corresponding larger images.
Do not use image maps (i.e., images with multiple clickable regions). Their content cannot be indexed by any search engine, their links cannot be indexed by some search engines, and they are not accessible to screen readers for the visually impaired and text-only browsers.
Multimedia (Audio and Video Files) and Flash
Use HTML5 native multimedia to play audio and video files.
Supply multiple file formats to accommodate the full array of HTML5-compatible browsers.
For older browsers that do not support HTML5 native multimedia, include fallbacks such as Flash player and embedded files uploaded to third-party sites (e.g., YouTube, etc.).
Remember that Flash is not supported by Apple devices, usually increases the time it takes for a Web page to load, and its content cannot be indexed easily by search engines. Consequently, only utilize Flash for unimportant content when it will not significantly increase page load time (i.e., never use Flash for page headers containing contact information and for navigation menus).
Never auto-play audio files and videos containing audio when a Web page loads. (Unsuspecting visitors may be startled or annoyed by the unexpected sound.)
Always mark mandatory fields.
Format forms so that they are easily viewed and used on smaller devices (e.g., position a field’s label above the field, not to its left).
Do not repeat the page’s navigation menu in its footer to avoid cluttering the display on small devices and increasing the effort and cost of maintenance.