Quoting from https://aaronparecki.com/oauth-2-simplified/
Single-page apps (or browser-based apps) run entirely in the browser after loading the source code from a web page. Since the entire source code is available to the browser, they cannot maintain the confidentiality of their client secret, so the secret is not used in this case. The flow is exactly the same as the authorization code flow above, but at the last step, the authorization code is exchanged for an access token without using the client secret.
Note: Previously, it was recommended that browser-based apps use the "Implicit" flow, which returns an access token immediately and does not have a token exchange step. In the time since the spec was originally written, the industry best practice has changed to recommend that the authorization code flow be used without the client secret. This provides more opportunities to create a secure flow, such as using the state parameter. References: Redhat, Deutsche Telekom, Smart Health IT.
I wanted to see how easy it would be to support this with WSO2 Identity Server without touching or patching the product code.
Turns out it wasn't that hard thanks to the extension points :)
Here's my attempt.