Backdrop blurred div is not covering entire screen

I have a div called overlay and then a container called signupContainer. I want the overlay div to cover the entire screen but be blurred in the background. This is my code:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-css -->

html, body {
    background-image: url('https://host.stcollier.repl.co/img.png');
    background-position: center;
    background-attachment: fixed;
    background-repeat: no-repeat;
    background-size: 1000px;
    background-color: rgba(255, 255, 255 0.5);

}

.blur {
    width: 100%;
    height: 100%;
    padding: 0;
    margin: 0;
    z-index: -99;
    backdrop-filter: blur(10px);
    position: fixed;
    top: 0;
    left: 0;
}

.signupContainer {
    padding-top: 75px;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    width: 100%;
    z-index: 99;
    color: white;
}

<!-- language: lang-html -->

<div class="blur"></div>

<div class="signupContainer">
  <h1>Sign Up</h1>
</div>

<!-- end snippet -->
Here's how that looks:

Notice how the backdrop filter blur is not covering the entire screen, but only the point to where other element is. Why is this and how can I fix this?

Just remove the html selector from the first rule, and then it will work. You're applying two background images unnecessarily and it seems that this is causing a wrong rendering.

body {
    background-image: url('https://host.stcollier.repl.co/img.png');
    background-position: center;
    background-attachment: fixed;
    background-repeat: no-repeat;
    background-size: 1000px;
    background-color: rgba(255, 255, 255 0.5);
}

It's because the backdrop position is relative so it's not covering the entire page.

you could use this:

.blur {
    position: absolute;
    width: 100vw;
    height: 100vh;
    padding: 0;
    margin: 0;
    z-index: -99;
    backdrop-filter: blur(10px);
    position: fixed;
    top: 0;
    left: 0;
}