Websites That Don't Work in Any Browser Other Than Internet Explorer
Creating a website that only works in Internet Explorer is a very bad business decision. More and more people are switching to other Web browsers like Chrome and Firefox. And if you insist on creating pages that only work in IE, you're deliberately limiting your market share.
Web Designs That are Written for Specific Browsers and Versions, and Say So
It's not good design or good customer service to try and drive your customers away. And putting a banner at the top of your page that reads "this page is best viewed in Internet Explorer" does just that. Customers don't want to be dictated to. They want to do their work, read their websites, and get on with their day. Changing to a different Web browser isn't going to happen simply because your site doesn't work as well in Firefox or Opera. And in fact, IE is losing it's majority in the browser market.
Using Cutting Edge Technology for No Good Reason
It can be very tempting to use the newest technology. But if you put something on your website that is not widely supported by the browsers that visit your site you could risk losing customers rather than gaining them. When you use advanced technology (such as new versions of CSS or HTML), you should use it sparingly and only for elements that are not critical to the page as a whole.
Sound or Video That Starts Automatically
I can't say it too often. Most people, even if they like sound on websites, don't like it to start automatically. They may have their own music playing or be in a location where sound from their computer is distracting or inappropriate. If your website got someone in trouble with their boss, they aren't going to be terribly interested in visiting it again. No matter how nice the music is. This is all true for video as well.
Sound or Video That Doesn't Have an Easy to Find Off Switch
If you have sound on your website, you should have a way to turn it on and off that's easy to find. This is especially true if you ignore the previous tip and have it start automatically. By making the controls easy to find and use, you are showing respect for your readers. And putting the controls at the very bottom of a long page does not make them easy to find, as most people will have hit the back button before they'll have found the controls. Put them at the top of your page.
Hidden or Disguised Navigation
Websites that people like to visit are ones that are easy to use. If you put your navigation in a strange place (at the bottom, for example) or disguise it as something else (such as with an image map), you'll just annoy your readers. Some people will tell you that a good design uses a "theme", but if your theme includes turning your "home" navigation link into a loaf of bread and your "about us" link into a photo of a fish, your navigation will be too hard to use and you'll lose your readers.
100% Flash Content
Flash is a great tool, but if you use it for 100% of your Web content and don't have any text alternatives you risk losing readers. There are many usability issues with Flash, the most severe being that it's not accessible. But many search engines don't index Flash content well, and customers don't like sites where the back button doesn't work, which is often the case with Flash websites. Many people just don't like Flash and will click away the minute they see it.
Flash Splash Screens
Even if you don't do all your content in Flash, many designers will create a splash page in Flash to show off their skills or just provide some animation. The problem is that they are often boring or to commercial for most customers. People come to websites because they want information. Having to wait through even a 30 second Flash animation can seem like they are waiting through a commercial. And "Tivo" on a Web page is the back button as they go to somoene else's site.
Flash Navigation
Whatever you do, don't use Flash for navigation. Use Flash everywhere else on your website, but if you use it for navigation you'll have trouble. Customers don't find it easy to use, and it's not accessible. Plus, search engines will have a much harder time navigating your site so they won't get your pages into their index.
100% Image Websites That Are Not Photo Galleries
Many print designers will build their website in a print tool like Photoshop or Illustrator. Then when they can't get it to look exactly like the image, they give up and just publish the image as the Web page. This is hard to maintain, difficult for your readers to use, and often slow to load. It's just not a good idea. If you're determined to design for print, then stick with print. Web designers need to be more flexible.
