base64url encoding for IIIF Content State

This demo shows the proposed mechanism for encoding a content state when sent as a request parameter. But can it be simpler? Shorter? Safer? What have we missed? Are there better ways?
Try out some strings below to experiment.

What is IIIF Content State? | Content State Specification | Source code for this experiment on GitHub

Simple JavaScript encoding
Unicode JavaScript encoding
encodeURIComponent JavaScript encoding (Content State 0.9)
No padding (Content State 0.9)

Client plaintext

Client encoded

Server plaintext

Server encoded