Light client
Checks one account's balance without trusting the server's word for it -- verifies the block header chain back to a hardcoded genesis hash, then checks a Merkle proof against it, entirely in this browser tab.
What this actually proves: that the balance a node reports for one account is consistent with a header chain that hash-links, block by block, back to a genesis hash written into this page's own source (view-source it if you don't trust us either). What it doesn't prove: that the header chain itself is the real one -- this checks integrity against the single endpoint you point it at, it doesn't (yet) cross-check that endpoint against independent peers. A compromised or lying RPC server could still feed a consistent-but-fabricated chain from genesis onward. Same starting trust model every SPV-style light client has; this is the "verify what one server told you" half, not the "which server should I even ask" half.