Check in JavaScript if an SSL Certificate is valid

What I've found up to now - it is possible with Firefox, don't know yet about other browsers:

https://developer.mozilla.org/En/How_to_check_the_security_state_of_an_XMLHTTPRequest_over_SSL

Take a look here: https://support.mozilla.org/pl/questions/923494

<img src="https://the_site/the_image" onerror="redirectToCertPage()">

This solution is tested and working in current versions of FF and Chrome (as of 2022):

<script> var sslCertTrusted = false; </script>
<script src="https://example.com/ssltest.js"></script>
<script> 
    if (!sslCertTrusted) 
    {
        alert('Sorry, you need to install the certificate first.');
        window.location.replace('http://example.com/cert_install_instructions/');
    }
    else
    {
        // alert('Redirecting to secure connection')
        window.location.replace('https://example.com/');
    }
<script>

You of course need to make your web server return this code under the URL https://example.com/ssltest.js:

sslCertTrusted = true;

I'm not exactly sure about the details. But I've seen similar technology used to detect adblocking etc. You may need to piggyback on the window object maybe, if the variable can't be modified by another script, but generally making the above proof of concept work is left as an exercise to the reader.

The straight answer is no. Javascript does not provide any means of validating certificates. This is a job left to the browser.

A better approach to this problem is from the server side. If you are controlling the site, than you can render down a variable on the page with information gleaned on the server side.

In .Net something like

var canSecure = <%= MySiteHasSsl ? "true" : "false" %>;
if (canSecure) {
    if (confirm("This site supports SSL encryption. Would you like to switch to a secure connection?")) {
        location.href = "https://mysite.com";
    }
}