JDK 8 uses the Javascript Engine Nashorn instead of Rhino in JDK 7 (and 6).
Therefore, 2 additional config lines are required to support JDK 8 in addition to JDK 7:

After making the following 2 config updates for bean validation my web app is running with WildFly and OpenJDK 8:

1) In modules/system/layers/base/sun/jdk/main/module.xml: add the following <path> declaration as the 2nd subtag inside <paths>, i.e. after the "com/sun/script/javascript" for JDK 7:

     <path name="jdk/nashorn/api/scripting"/>

2) In modules/system/layers/base/sun/jdk/main/service-loader-resources/META-INF/services/javax.script.ScriptEngineFactory add the following line as the 2nd service, i.e. as a alternative service for JDK 7:

     jdk.nashorn.api.scripting.NashornScriptEngineFactory