In light of reaching 15k followers on here:

AMA!

Dev at ethOS, a mobile OS that is crypto native- nf.td/markus, Technology brother

What's missing in the tech stack to make every DeFi frontend and/or every wallet software have a built-in Ethereum light client in its JavaScript code?

Not a lot!
Here is a light client proxy that has Web-Assembly bindings for dapps and wallets: https://github.com/a16z/Helios

Made by @ncitron.eth :)

building open source public goods @a16zcrypto

Contributor @ /walletbeat.

Censorship resistance requires privacy.

@polymutex.eth if you end up building something with Helios feel free to reach out. Always happy to help out a fellow light client enjoyer!