User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/118.0.0.0 Safari/537.36

Steps to reproduce:

First example:

1. In Firefox, open up this link: https://codepen.io/yihuiliao/pen/qBLzzxO
2. Inspect the accessibility properties of the progress bar
3. Notice how the progress bar has value="50" even when it is indeterminate and has no value set in the code

Second example:

1. In Firefox, open up this link: https://codepen.io/yihuiliao/pen/QWzXXxV
2. Click on the button and inspect the accessibility properties of the button as the progress circle is spinning
3. Notice how the name of the button becomes "Loading...50 Click me" and how the progress circle has a value="50" even though it is indeterminate and has no value set in the code
4. Turn on a screenreader and click on the button, while the announcement depends on the screenreader used, you will hear "50" being announced at some point.

Actual results:

In the second example with NVDA, because the indeterminate progress circle has a value of 50, this becomes a part of the accessible name of the button. When clicking on the button to start the progress circle, "Loading....50 Click me" is announced (alongside some other stuff but the important part is the "Loading...50 Click me".

In other browsers such as Chrome, this is not an issue. In both the first and second example, the progress bar do not have a value of 50. If you inspect the accessibility properties, the progress bar's have Value description: "". When you run NVDA on Chrome with the second example, you will hear "Loading...Click me". At no point is "50" ever announced.

Expected results:

An indeterminate progress bar should not have a value="50".

I can reproduce the issue on Windows 10x64 and macOS 12 with Firefox 120.0a1 (2023-10-18), 119, 118.0.2 and 115.3.1esr.

Using the first example's steps I also managed to make a regression range with mozregression but being so old I don't know which is the issue that caused this. On good builds the value was empty and bad ones the value was 50:
Pushlog: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=c8b9ae708d8e1805bd68f3deb3c0fe5cd9f9e1c5&tochange=5c424bc57454d9d2f6d64c02aaf5d31c04f9ad2f

Moving this to Devtools: Accessibility tools as well. Feel free to change it If needed. Thank you!