webxml attribute is required with Servlet 3.0

By default maven-war-plugin will fail if it can’t find web.xml, see here. If you are creating Maven project using latest archetype, you will get this parameter set to false:

  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-war-plugin</artifactId>
        <configuration>
          <failOnMissingWebXml>false</failOnMissingWebXml>
        </configuration>
      </plugin>
    </plugins>
  </build>

If you want to use web.xml instead of annotations, just create WEB-INF/web.xml and define servlet there, see Book of Vaadin for detailed instruction.