how to validate the browser zoom?

If what you want is to make sure that when you reload the page you will have a 100% zoom, you could use the property transform: scale of CSS, with which you can set the scale that your page will have (a scale of 1 will mean that you will have a zoom of 100%, a scale of 2, 200%, and so on).

I leave you a small example with a scale of 3 (300%):

body{
   transform: scale(3);
   transform-origin: 0 0;
}

<p id="conZoom">Esto es una prueba</p>
<p id="sinZoom">Esto es una prueba</p>
<img src="https://media.licdn.com/mpr/mpr/shrinknp_800_800/AAEAAQAAAAAAAARpAAAAJDMzZGRhNGMwLTU4YmMtNDdmZi1hMjU5LWIwYTViMjdlNWJmOQ.png">

In your case you should simply indicate that you want a scale of 1 to obtain 100% zoom.

Why do I use the property transform: scale instead of the property zoom CSS?

Because the transform: scale property supports all popular browsers, including Internet Explorer 11. You can see the compatibilities with this property here .

However, the zoom property does not support Firefox (without using prefixes). You can see it here .

In case you want to support versions of Internet Explorer lower than 9, you could use the property zoom failing.