http://webassembly.org/docs/rationale/ пишет: We started with an AST and generalized to a structured stack machine. ASTs allow a dense encoding and efficient decoding, compilation, and interpretation. The structured stack machine of WebAssembly is a generalization of ASTs allowed in previous versions while allowing efficiency gains in interpretation and baseline compilation, as well as a straightforward design for multi-return functions.

http://webassembly.org/docs/rationale/ пишет: Multiple Return Values

WebAssembly’s MVP does not support multiple return values from functions because they aren’t strictly necessary for the earliest anticipated use cases (and it’s a minimum viable product), and they would introduce some complexity for some implementations. However, multiple return values are a very useful feature, and are relevant to ABIs, so it’s likely to be added soon after the MVP.