The 5 biggest, most important NO-NOs of Web Designing are:
5) Don’t use flash just because you can! There’s always a chance that some people do not have plug-ins that are capable of playing what you’ve put on your site with flash, which means that they miss it and that is not what you want. This is not to say, don’t use flash, use it, but in discretion!
4) Fonts: use normal, plain fonts, they ALWAYS work the best– they are visible and most importantly, available everywhere. The problem is that, if a fancy font is used, it may or may not be available in every computer. Therefore, the chance of changing to a font you didn’t want, by default can be sidestepped. If a fancy font is a must, it makes sense to use it in an image where it will be visible as it is.
3) The image should match the content: do not have disconnected images. Make sure that the images on the site will ensure that the visitor will understand what it is about even without reading it. Using false messages to attract visitors is wrong and must not be encouraged.
2) Size of the picture/image itself: just because images make the site look better, do not fill your entire site with them. The bigger the images, the longer they will take to load, and the longer they take, the chances of the user staying on the page decreases.
1) The most important of the lot: KISS. Keep It Simple, Stupid! Don;t make it unnecessarily complicated, it’s not worth it!