Is this OK or not really? If I develop a website template, CSS/HTML/JS, is it acceptable to post that as a code sample to be reviewed as an external URL?
Generally, no. Per the faq:
Make sure you include your code in your question
This site is for code reviews, which are hard to do when the code is behind a link somewhere out there on the internet. If you want a code review, you must post the relevant snippets of code in your question. It is fine to post a "see more" link (though, do be careful — very few reviewers will be willing to click through and read thousands of lines of your code), but the most important parts of the code must be placed directly in the question.
However you should make sure to include at least the important parts of the code in your question, not just post a link to the site.