Reference

Reference collections covering HTML and CSS validation issues, DNS record types, and networking fundamentals — explanations, examples, and links to the source specs.

HTML Validation IssuesWhy each HTML Validator issue matters and how to fix it CSS Validation IssuesWhy each CSS Validator issue matters and how to fix it DNS Record TypesA, AAAA, CNAME, MX, NS, SOA, TXT, PTR explained NetworkingIP addresses, IPv4, IPv6, subnets, and CIDR notation HTML EntitiesNamed and numeric character reference lookup HTTP Status CodesWhat each status code means and when it's returned