As of July 2024, you can use the following code to determine if a browser is on visionOS or not. (This does not identify browser brands; I only checked major browsers.)

function isVisionOS () {
  return navigator.userAgent.includes("(Macintosh;") &&
    !!navigator.xr &&
    document.ontouchstart !== undefined;
}

Breakdown

The function is using 3 criteria.

1. Whether the user agent string contains "(Macintosh;" or not.

Safari on macOS, iPadOS, visionOS has user agents like following.

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/18.0 Safari/605.1.15

By using the following code, we can determine if the browser is on macOS, iPadOS, or visionOS.

navigator.userAgent.includes("(Macintosh;")

2. Whether the browser supports XR or not.

According to the MDN page, Safari for macOS, iPadOS does not support XR. However, Safari on visionOS supports.

You can check if XR is supported with the following code:

!!navigator.xr

So far, it seems like criteria No.1 and No.2 are enough to identify visionOS. However, Chrome on macOS supports XR. So we need No.3.

3. Whether it is a touch device or not.

Finally, we can exclude macOS by using the following code.

document.ontouchstart !== undefined