Just really something I've been noticing as I read tutorials for my new application I'm developing, the 10 (ish) things I see most (PHP) coders doing, which they shouldn't in Web Dev/Design.
4. Don't use <table> or <font> tags. They're deprecated for a reason, in 1999 it was fine to use tables to lay out your content, but since the advent of CSS, it's often easier now to use divs and classes, or even HTML5 Elements combined with CSS. (Switch To HTML5) The same can be said for adding styling to your text.
5. Don't use <style>. It's horrible practise. For a development site just to see how it looks and to tweak, it's perfect, but in a production site it's not really an option unless all other options are unavailable.
6. Don't make your navigation cluttered. This one is more web design rather than web development. People need to be able to navigate your site without needing a phD in computing. If an 8 year old can find the "log in" link after less than a minute, you're good.
7. Add a search feature. If someone can't find something on your site, and there is no "Search" box, you've just lost a visitor. If all of your visitors are potential customers, then you just lost money. If you don't know how to code, it's easy, Google offers a free embeddable search engine for your website with CSE.
8. Don't rely on images to display your most important content. I see web hosting companies doing this mostly, some text, then their price as an image. Your price is the most important part of the page, and if you display it as an image, someone with a slow connection will possibly not see it.
9. Don't make a crap web design, simple as that. Don't use excessive bright colours, Don't use a million and one different fonts, no comic sans for the love of god, try to keep it uniform throughout. People like consistency.
10. Don't spam it up with ads! If you have a sidebar, use it for content, not advertisements.
11a. PHP: Don't use shorthand. It's the same as styling using the <style> tag. <?= ?> tags only save a tiny bit of typing, so don't be lazy, do it right.
11b. PHP OOP: Don't edit the core controller!! If all of your controllers extend "Controller", please don't edit "Controller".. Any errors you make in that file are passed to all the Controllers extending that class, and it makes debugging a pain. Please don't tell users to edit the core "Controller" class if you're writing a tutorial, it's horrendously irresponsible.
In conclusion, just think it out before you do it, consider the implications of putting that there, and if you don't know what you're doing, I suggest you learn before trying to put a website/Web Design together. Paint and Notepad a web designer does not make.