Q: Why is it possible to write HTML inside Scalatra? Take the following example : 
package com.example.app

import org.scalatra._
import scalate.ScalateSupport

class MyServlet extends ScalatraServlet with ScalateSupport {

  get("/") {
    <html>
      <body>
        <h1>Hello, world!</h1>
        Say <a href="hello-scalate">hello to Scalate</a>.
      </body>
    </html>
  }
}

Is this a DSL? i am wondering about the mechanism of how this work.

A: Scala natively supports xml syntax and, by extension, xhtml. So no DSL, just a language feature.

A: Scala has support of XML literals on language level.

Scala's XML literal syntax is actually sugar for a series of Elem and
  Text instantiations

