The HTML Template Language (HTL), formerly known as Sightly, has been introduced with Adobe Experience Manager 6.0 and takes the place of JSP (JavaServer Pages) as the preferred and recommended server-side template system for HTML.
This is the HTL Specification that defines the syntax and the behavior of the language.
A Java-based reference implementation has been donated to the Apache Sling project and can be checked out from https://github.com/apache/sling/tree/trunk/bundles/scripting/sightly.
The Technology Compatibility Kit can be run on implementations to check their compliance with the specification.
An implementation of version 1.2 of the language specification is available in AEM 6.2.