a. Leaving the color of visited links the same - this is really not helpful for users when they visit a website. You should be able to identify which sites you've already been in. Sure, this mistake wouldn't be that grave when you only have a few links in your site. But what if you've got hundreds? You can't really expect your site visitor to write down the URL of each and every link they've clicked. That would be a waste of time and energy. But if you addressed this during your actual designing process, you'd be saving a whole lot of people the trouble.
b. Non-arranged array of text - can you just imagine browsing through a site with text from top to bottom, left to right. It's not just un-aesthetic, but it's too much text for both the eyes and the mind to handle. If you could just make use of bullets and proper indention and spacing, the whole page would look a lot friendlier to the eyes and a whole lot understandable to the mind. Highlighting specific keywords would also help users understand the point that you're trying to get across. And pictures bring a lot of life into your page as well. Actually, a window filled with pictures is more expressive than a screen filled with blubbers of texts.
c. Not opening to a new window or tab when clicking a link - though browsers always have back-buttons, users still appreciate it if a new window or tab is opened when they click a link. This will give them a good navigation during their internet browsing. It will also help users have an easier tracking access to the sites they've previously visited. If your goal is to make sure that your site visitors enjoy your site, then try to make sure that you don't repeat this mistake.
There are still a whole lot of other mistakes done by beginners and advanced designers alike. And it's good if you visit other sites yourself and study what makes you hate their sites or what makes you enjoy their pages. This will help you build ideas and restrictions for your web design.
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