Good error reporting is essential when building and debugging templates. This page explains how to find errors in your template and CSS files.
CSS errors (usually LESS processing errors) will be printed at the bottom of the compiled site.css file. Access it by browsing to http://yoursitename.squarespace.com/site.css and scroll to see error messages. If your site suddenly appears to be rendering with no CSS styles, it is a good time to check your site.css.
Template Structure Errors
Template file errors (mostly errors in misformatted template configuration files) can be viewed in the Developer tab.
Squarespace also reports JSONT related errors on your front end site directly.