Whenever possible, use a single domain & subdomain
It’s hard to argue this given the preponderance of evidence and examples of folks moving their content from a subdomain to subfolder and seeing improved results (or, worse, moving content to a subdomain and losing traffic). Whatever heuristics the engines use to judge whether content should inherit the ranking ability of its parent domain seem to have trouble consistently passing to subdomains.
The more readable by human beings, the better
It should come as no surprise that the easier a URL is to read for humans, the better it is for search engines. Accessibility has always been a part of SEO, but never more so than today, when engines can leverage advanced user and usage data signals to determine what people are engaging with vs. not.
Keywords in URLs: still a good thing
It’s still the case that using the keywords you’re targeting for rankings in your URLs is a solid idea. This is true for several reasons.
Multiple URLs serving the same content? Canonicalize ’em!
If you have two URLs that serve very similar content, consider canonicalizing them, using either a 301 redirect (if there’s no real reason to maintain the duplicate) or a rel=canonical.
Shorter > longer
Shorter URLs are, generally speaking, preferable. You don’t need to take this to the extreme, and if your URL is already less than 50-60 characters, don’t worry about it at all. But if you have URLs pushing 100+ characters, there’s probably an opportunity to rewrite them and gain value.
Match URLs to titles most of the time (when it makes sense)
This doesn’t mean that if the title of your piece is “My Favorite 7 Bottles of Islay Whisky (and how one of them cost me my entire Lego collection)” that your URL has to be a perfect match.
Including stop words isn’t necessary
If your title/headline includes stop words (and, or, but, of, the, a, etc.), it’s not critical to put them in the URL. You don’t have to leave them out, either, but it can sometimes help to make a URL shorter and more readable in some sharing contexts. Use your best judgement on whether to include or not based on the readability vs. length.
Limit redirection hops to two or fewer
If a user or crawler requests URL A, which redirects to URL B. That’s cool. It’s even OK if URL B then redirects to URL C (not great—it would be more ideal to point URL A directly to URL C, but not terrible). However, if the URL redirect string continues past two hops, you could get into trouble.
Hyphens and underscores are preferred word separators
Notably missing (for the first time in my many years updating this piece) is my recommendation to avoid underscores as word separators in URLs. In the last few years, the search engines have successfully overcome their previous challenges with this issue and now treat underscores and hyphens similarly.
Keyword stuffing and repetition are pointless and make your site look spammy
Check out the search result listing below, and you’ll see a whole lot of “canoe puppies” in the URL. That’s probably not ideal, and it could drive some searchers to bias against wanting to click.