Not all HTML tags are of the same structure. While most elements require an opening tag, a closing tag, and contents, some elements - known as void elements - only require an opening tag as they themselves do not contain any elements. This topic explains and demonstrates the proper usage of void elements in HTML
DEX means Android app's (APK) executable bytecode files in the form of Dalvik Executable (DEX) files, which contain the compiled code used to run your app.
The Dalvik Executable specification limits the total number of methods that can be referenced within a single DEX file to 65,536 (64K)—including Android framework methods, library methods, and methods in your own code.
To overcome this limit requires configure your app build process to generate more than one DEX file, known as a Multidex.