W3C Markup Validation Service - Validates HTML markup of web pages, one at a time.
OpenSSL - OpenSSL is a free and open source software cryptography library that implements both the Secure Sockets Layer (SSL) and the Transport Layer Security (TLS) protocols, which are primarily used to provide secure communications between web browsers and …
Total Validator - Total Validator can check your website to help ensure that it is accessible, uses valid HTML and CSS, has no broken links, and is free from spelling mistakes.
Ensighten - Ensighten provides enterprise tag management solutions that enable businesses manage their websites more effectively.
Rocket Validator - Site-wide Accessibility and HTML validation made easy. With a single click, Rocket Validator will check thousands of web pages on your site for Accessibility and HTML conformance, producing a detailed, actionable report on seconds.
Let's Encrypt - Let’s Encrypt is a free, automated, and open certificate authority brought to you by the Internet Security Research Group (ISRG).