Reference
Reference collections covering HTML/CSS validation, DNS records, networking, and browser/crawler user agents, with links to 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
User AgentsBrowser UA strings, and identifying Googlebot/Bingbot/AI crawlers
HTML EntitiesNamed and numeric character reference lookup
HTTP Status CodesWhat each status code means and when it's returned