Parallax template has stopped working properly on w3 schools website

[lizzyfiesta]

https://www.w3schools.com/w3css/tryw3css_templates_parallax.html

I used the template above a few months ago to create a parallax website.  The background scrolled at a slower speed to the rest of the website, which was perfect.  All of a sudden, the backgrounds have stopped scrolling, the other text just scrolls over the top of the backgrounds and they stay still.   I thought I'd made a mistake but when I went back to the original template (link above), I discovered that the w3schools site no longer works properly either.  Something has changed in the browsers I guess, does anybody know what it is and why?  I've used Chrome, Firefox and Safari to test.  I've searched for other parallax websites and can find sites that scroll properly, i.e. the background scrolls at a slower speed than the rest of the site.  I'm very confused.

[davej]

I get a 404 error on that page so I suspect the site designers are changing things. You might start here...

https://www.w3schools.com/howto/howto_css_parallax.asp

[dsonesuk]

It should end with ext .htm not html.