How is WebAssembly cross-platform when Assembly is not

• WebAssembly is like Assembly, except that
  • it runs cross-platform
  • it runs isolated
  • it runs on browsers
• But how is WASM fast and cross-platform simultaneously?
  • Does it pack multiple precompiled binary executables for multiple CPU vendors, like Apple's Universal Binary?
  • But then, whenever a new type of CPU gets announced, they wouldn't be able to run existing WASMs
• WebAssembly's runtime environments (RE) are low-level virtual stack machines (akin to JVM or Flash VM)
• Seems like WASM is closer to intermediate Java Byte Code instead of the genuinely low-level Assembly.
  • But then, why is it faster?
  • JS Interpreter can skip the parsing
  • It can ship in a much more compact file format
• WASM is just like Java Byte-code. Java Byte-code is cross-platform when machine code for a real CPU is not. It's input for a JIT compiler that targets whatever real CPU. Peter Cordes
• WASM defines its own _CPU standards and Assembly. WebAssembly Core Specification.
• To run WASM, the browser must still compile the WASM code into ASM code when executing; in that way, it is much slower than WASM.
• However, WASM is designed similarly to ASM. Therefore, compiling WASM to ASM targeting x86, ARM, and RISC-V, is comparably easy, and existing compilers emitting ASM can also emit WASM with a reasonable modification.
• What is the relationship between WebAssembly and Assembly?
• It is a specific purpose Assembly.
• It's designed with a specific abstract machine that would be expensive to implement in hardware.

References

1. WebAssembly
2. How is WebAssembly cross-platform when Assembly is not? - Stack Overflow