Here are my favorite resources for building and troubleshooting web pages I maintain. All are free unless otherwise noted.
Google Webmaster Tools. Lots of diagnostics on your web sites.
Google Analytics. Great tool for tracking activity and behavior.
Page Speed Insights. Checks to see if pages is consdered "mobile-friendly" by Google and gives tips for fixing it. This page also provides analysis and tips for page loading speed.
Response Web Design Quick Tips
W3Schools. Great site for learning CSS, Javascript, Boostrap, and other languages and frameworks.
Convert Video Files. Great free site to replace those old SWF (and other video) formats with more broadly supported formats.