Here are my favorite resources for building and troubleshooting web pages I maintain. All are free unless otherwise noted.
Google Search Console. Lots of diagnostics on your web site. Everyone should use this.
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.
Convert Video Files. Great free site to replace those old SWF (and other video) formats with more broadly supported formats.