Our recommendation for templating, particularly for HTML is Thymeleaf. Thymeleaf is the default template engine in Spring and Spring Boot provides excellent support through a corresponding starter and auto-configuration:
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-thymeleaf</artifactId>
</dependency>
Of course, you can also use other templating engines such as Freemarker or Velocity. Spring Boot also provides support for some of them out-of-the-box.
Note that JSP can of course also be used with Girders 6, but it is not recommended anymore. Spring Boot does support JSP, but there are several known limitations, particularly in combination with embedded servlet engines.