To improve build performance, applications with Dojo dictate which files to include in the build using the build-dojo.xml file. By default, it is optimized for mobile development. If you'd like to include the HorizontalSlider, the easiest way is to uncomment <include name="dojo/dijit-layer.js.compressed.js"/>, but leave the <include> elements after that commented out. That will give you a patternset of "dojo/dojo.js.compressed.js", "dojo/core-web-layer.js.compressed.js", and "dojo/dijit-layer.js.compressed.js".
If you've used the Palette to drag and drop the HorizontalSlider, it should have added references to dijit.css and claro.css in the application's markup. So these resources will need to be added to your application to style the widget. You could do this manually by copying the dijit/themes/claro/ directory and dijit/themes/dijit.css file into your application. But as we saw, the build-dojo.xml file automates adding these things to the build. In your build-dojo.xml file, find the patternset dojo.resources.loose-modules. And add the following includes:
<include name="dijit/themes/claro/**"/>
<include name="dijit/themes/dijit.css"/>
Save the build-dojo.xml file.
Now you need to update your application's JS file. In wlCommonInit, the layer files are required. There is an array the includes what you would need for mobile development. You will need to add another layer to the array: "dojo/dijit-layer". So it should look something like:
require([ "dojo/core-web-layer", "dojo/mobile-ui-layer", "dojo/mobile-compat-layer", "dojo/dijit-layer" ], dojoInit);
Save your application's JS file and preview your application as you normally would. This should fix the 404 problems and render the HorizontalSlider widget.