Every web designer should follow a list of principles to make sure that sites they make for their clients are effective and top-notch.
Will somebody who is not familiar with websites or computers be able to navigate the website without any problems at all? Make sure all users are able to find the key information that they need. This usually means putting yourself in a computer novice’s shoes or grabbing a few test subjects and watch them navigate the site to look for key information.
Never assume that only one type of browser will be used to access the site. Be sure to test the site in all current versions of major browsers on Mac (Firefox, Safari, Chrome) and Windows (Internet Explorer, Firefox, Chrome). Don’t forget those mobile browsers as well!
Load time is important for visitors’ attention but also (as of 2011) for Google’s search results algorithm.
Make sure every link on the website works and that nothing is broken before handing the site over to a client. Check internal links as well as external links that go to other sites. If there is a link to a PDF document be sure to also provide a link to a PDF reader (Adobe Reader) just in case the user doesn’t have the software.
ALT tags are used for blind visitors but they also help describe what images are for search engines which will help you in the SEO department.