In my years of working with web pages, I've developed some opinions on what makes a courteous and ethical website. Here are some of them, both big picture and nitty-gritty tips on how to be a good web citizen through the pages you author:
Write proper HTML
This is not only the right thing to do, but it will be helpful as standards evolve. Your features are more likely to degrade gracefully when new rules come into force if you wrote them correctly in the first place. Plus, the types of devices that can display webpages are multiplying and you want your content to be readable on these phones, ebook readers, text-to-speech devices, etc. Keeping up with the current standards helps here, since they are designed to make web content device-neutral as much as possible.
Validate your pages, which is an easy way to find out if they're using current standards (check the resources under the Accessibility and Validation heading on the Resources tab). And keep up with the news at W3Schools.
Give credit where credit is due
"Borrowing" graphics, code, and layout ideas from other Web pages is an accepted emergent "tradition" of the medium. But, out of courtesy as well as honesty, and also in the spirit of helping others find "good stuff," credit your borrowings whenever possible. Also, be careful with your borrowing. Make sure the intellectual property you're lifting isn't proprietary. Looking for a Creative Commons designation is one way to do this, or check for restrictions in a site's "About" page. If in doubt, ask for permission.
I'm sure you've been to pages on the Web that looked great, but you were reluctant to cite or recommend them because you had no idea who had put them up. Don't let that happen to your readers. If you don't put some kind of identifying footer on each page (you might want to include a copyright notice—use © to make this sign: ©), you should put a link on every page back to some main page from which a reader can tell who you are and what kind of site they're visiting. You should also have a way for readers to contact you (maybe a mailto link) somewhere in your pages. See more details under the 
One of my biggest pet peeves about many web pages is their lack of a date notice. Do all your readers, especially your repeat "customers," a favor and include a last updated note on your pages. Many kinds of content, in addition to the obvious like schedules, are time-sensitive to some degree. On some of the resource pages I've consulted in updating this guide, my only indication of the datedness of a page is when they mention 