I managed to get a pure HTML + javascript version working. I got most of the code from a blog article, but can’t find it now 
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title></title>
</head>
<body>
<div>
<div>
<a href="#" id="start">Click to Sign In</a>
<div id="token">
<h2>Access Token</h2>
<div id="access_token"></div>
</div>
<div id="error">
<h2>Error</h2>
<div id="error_details"></div>
</div>
</div>
</div>
<script src="j.js"></script>
</body>
</html>
// Configure your application and authorization server details
var config = {
client_id: "xxx",
redirect_uri: "https://localhost:44336/index.html",
authorization_endpoint: "https://dev-xxx.okta.com/oauth2/default/v1/authorize",
token_endpoint: "https://dev-xxx.okta.com/oauth2/default/v1/token",
requested_scopes: "openid"
};
// PKCE HELPER FUNCTIONS
// Generate a secure random string using the browser crypto functions
function generateRandomString() {
var array = new Uint32Array(28);
window.crypto.getRandomValues(array);
return Array.from(array, dec => ('0' + dec.toString(16)).substr(-2)).join('');
}
// Calculate the SHA256 hash of the input text.
// Returns a promise that resolves to an ArrayBuffer
function sha256(plain) {
const encoder = new TextEncoder();
const data = encoder.encode(plain);
return window.crypto.subtle.digest('SHA-256', data);
}
// Base64-urlencodes the input string
function base64urlencode(str) {
// Convert the ArrayBuffer to string using Uint8 array to convert to what btoa accepts.
// btoa accepts chars only within ascii 0-255 and base64 encodes them.
// Then convert the base64 encoded to base64url encoded
// (replace + with -, replace / with _, trim trailing =)
return btoa(String.fromCharCode.apply(null, new Uint8Array(str)))
.replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '');
}
// Return the base64-urlencoded sha256 hash for the PKCE challenge
async function pkceChallengeFromVerifier(v) {
hashed = await sha256(v);
return base64urlencode(hashed);
}
// Initiate the PKCE Auth Code flow when the link is clicked
document.getElementById("start").addEventListener("click", async function (e) {
e.preventDefault();
// Create and store a random "state" value
var state = generateRandomString();
localStorage.setItem("pkce_state", state);
// Create and store a new PKCE code_verifier (the plaintext random secret)
var code_verifier = generateRandomString();
localStorage.setItem("pkce_code_verifier", code_verifier);
// Hash and base64-urlencode the secret to use as the challenge
var code_challenge = await pkceChallengeFromVerifier(code_verifier);
// Build the authorization URL
var url = config.authorization_endpoint
+ "?response_type=code"
+ "&client_id=" + encodeURIComponent(config.client_id)
+ "&state=" + encodeURIComponent(state)
+ "&scope=" + encodeURIComponent(config.requested_scopes)
+ "&redirect_uri=" + encodeURIComponent(config.redirect_uri)
+ "&code_challenge=" + encodeURIComponent(code_challenge)
+ "&code_challenge_method=S256"
;
// Redirect to the authorization server
window.location = url;
});
// Parse a query string into an object
function parseQueryString(string) {
if (string == "") { return {}; }
var segments = string.split("&").map(s => s.split("="));
var queryString = {};
segments.forEach(s => queryString[s[0]] = s[1]);
return queryString;
}
// Make a POST request and parse the response as JSON
function sendPostRequest(url, params, success, error) {
var request = new XMLHttpRequest();
request.open('POST', url, true);
request.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=UTF-8');
request.onload = function () {
var body = {};
try {
body = JSON.parse(request.response);
} catch (e) { }
if (request.status == 200) {
success(request, body);
} else {
error(request, body);
}
}
request.onerror = function () {
error(request, {});
}
var body = Object.keys(params).map(key => key + '=' + params[key]).join('&');
request.send(body);
}
// Handle the redirect back from the authorization server and
// get an access token from the token endpoint
var q = parseQueryString(window.location.search.substring(1));
// Check if the server returned an error string
if (q.error) {
alert("Error returned from authorization server: " + q.error);
document.getElementById("error_details").innerText = q.error + "\n\n" + q.error_description;
}
// If the server returned an authorization code, attempt to exchange it for an access token
if (q.code) {
// Verify state matches what we set at the beginning
if (localStorage.getItem("pkce_state") != q.state) {
alert("Invalid state");
} else {
// Exchange the authorization code for an access token
sendPostRequest(config.token_endpoint, {
grant_type: "authorization_code",
code: q.code,
client_id: config.client_id,
redirect_uri: config.redirect_uri,
code_verifier: localStorage.getItem("pkce_code_verifier")
}, function (request, body) {
// Initialize your application now that you have an access token.
// Here we just display it in the browser.
document.getElementById("access_token").innerText = body.access_token;
// Replace the history entry to remove the auth code from the browser address bar
window.history.replaceState({}, null, "/");
}, function (request, error) {
// This could be an error response from the OAuth server, or an error because the
// request failed such as if the OAuth server doesn't allow CORS requests
document.getElementById("error_details").innerText = error.error + "\n\n" + error.error_description;
});
}
// Clean these up since we don't need them anymore
localStorage.removeItem("pkce_state");
localStorage.removeItem("pkce_code_verifier");
}