I worked on JVM bytecode for a significant number of years before working on Wasm. JVM bytecode verification is non-trivial, not only to specify, but to implement efficiently. In Java 6 the class file format introduced stack maps to tame a worst-case O(n^3) bytecode verification overhead, which had become a DoS attack vector. Structured control flow makes Wasm validation effectively linear and vastly simpler to understand and vet. Wasm cleaned up a number of JVM bytecode issues, such as massive redundancy between class files (duplicate constant pool entries), length limitations (Wasm uses LEBs everywhere), typing of locals, more arithmetic instructions, with signedness and floating point that closer matches hardware, addition of SIMD, explicit tail calls, and now first-class functions and a lower-level object model.