> knowingly accessed a computer without authorization or exceeding authorized access
The CFAA requires knowingly exceeding authorized access or doing so intentionally. If, in fact, you don’t know access is unauthorized, you’re not liable for a CFAA violation. So you can go blissfully through life doing what you, in good faith, may reasonably guess the web site owner intends you to do. You don’t need the rules “deliniated” just like for any other kind of property.
However, there is what you say about what you knew, and what a jury may reasonably infer about what you knew. If you say you didn’t know accessing port 80 was unauthorized, the jury will probably buy it, because port 80 is expected to host publicly available web services. However, if you take a URL and start manually fiddling with numbers, and you see that it yields credit card information, the jury probably won’t believe you if you say you didn’t know that access was unauthorized.[1] Similarly if you access a resource, and the web site owner tells you not to do it, or bans your IP, now you know. CFAA prosecutions are almost invariably based on a scenario where it is abundantly clear that someone knew she wasn’t supposed to be doing what she was doing.
[1] But, say you’re accessing a page programatically and there is a bug in your code and you access some URL that yields credit card information, which you promptly delete because you are acting in good faith. There is no CFAA violation because you had neither knowledge or intent. And to my knowledge, nobody has been convicted of violating the CFAA in such a situation.