There are numerous page validation and similar tools available online.
Many of these are great ways to check your site for problems, but sometimes the errors or warnings they give can be a bit scary.
Here's a list of resources to resolve common errors and warnings from popular validators:
Google PageSpeed Insights
- Official PageSpeed Insights tool
- How can I resolve Google PageSpeed Insights warnings?
- Need for speed: How much weight should you give Google's PageSpeed Insights?
Google Rich Snippets
- Official Facebook Crawler debugging tool
- Specify what image/text is shown when someone shares your site on Facebook
- Official Twitter Card Validator tool
- Specify what image/text is shown when someone shares your site on Twitter
Page Not Found 404 Validators
- Why am I getting 404 errors on product pages for files with color names?
- How to set up redirects in Shopify