Move your html file into the same location as your java class. The html code is looking for your java class in JRE System Library and that is not the location of your class.
You could also place you html code in the Lesson 10 folder and use the following in your html file replace the APPLET code with your class information.
<APPLET code="com.practicaljava.lesson11.TicTacToeApplet" codebase="./bin" width=350" height="400" />
The code is basically saying that the applet code is at com/practicaljava/lesson11/TicTacToeApplet.class and the place to look is /bin.
hope this helps