One issue I encountered was due to a parent element having the CSS property transform: scale(1);. This caused problems with elements using the fixed position inside it. Once I removed the transform property, everything functioned as expected.

This issue appeared consistently across different browsers I tested (Chrome, Safari), indicating that it may not be related to any specific web standard.

(In this scenario, the element in question was a popup transitioning from scale(0) to scale(1))

When a parent element includes a transform property, unexpected behavior may occur. You can try fixing this issue by commenting out the transforms in the code below:

-webkit-transform: translate3d(0,0,0);
transform: translate3d(0,0,0);

Make sure to verify that you haven't activated the property backface-visibility for any of the parent elements, as it can interfere with the functionality of position: fixed. In my case, I had implemented a CSS3 animation library...

Check out this Interactive Demo on jsFiddle

When working with CSS properties like fixed or absolute, it's recommended to define values for top, bottom, left, and right (or a combination of them).

Avoid setting the height directly on the main element; let the browser determine its height based on the top and bottom properties.

.header{
    position: fixed;
    background-color: #f00;
    height: 100px;
    top: 0;
    left: 0;
    right: 0;
}
.main{
    background-color: #ff0;
    position: fixed;
    bottom: 120px;
    left: 0;
    right: 0;
    top: 100px;
    overflow: auto;
}
.footer{
    position: fixed;
    background-color: #f0f;
    height: 120px;
    bottom: 0;
    left: 0;
    right: 0;
}

Highlighted by some, specific CSS styles applied to the ancestor element can interfere with the functionality of position: fixed. I encountered this issue specifically with the backdrop-filter property.

My issue stemmed from including a CSS property for perspective in the main container's styling

container { perspective: 800px; }

Solved

#sidebar { position: absolute; }

This may seem like an outdated issue, but in my situation, the culprit was actually the layout value of the css contain property within the parent element. The framework I'm using for hybrid mobile applications heavily relies on this property in many components.

Here is an example:

.parentEl {
    contain: size style layout;
}
.parentEl .childEl {
    position: fixed;
    top: 0;
    left: 0;
}

To resolve the problem, simply remove the layout value from the contain property, and the fixed content should function properly!

.parentEl {
    contain: size style;
}

If you're experiencing trouble with navbars not staying at the top, I discovered that the problem arises when any element within the parent container of the positon: fixed; element has a width greater than 100%, causing horizontal scrollbars to appear.

To fix this, simply adjust the 'parent element' to have overflow-x: hidden;

One issue is that the width is not set and there is no content in the divs. Another issue arises from the way HTML hierarchy works: when all three are fixed, the content is placed on top of the header because they are both fixed. To solve this, you can declare a z-index on the header, but it's recommended to leave it relative for normal scrolling behavior.

For a mobile-first approach, check out this FIDDLE.

HTML

<header class="global-header">HEADER</header>

<section class="main-content">CONTENT</section>

<footer class="global-footer">FOOTER</footer>

CSS html, body { padding: 0; margin: 0; height: 100%; }

.global-header {
    width: 100%;
    float: left;
    min-height: 5em;
    background-color: red;
}

.main-content {
    width: 100%;
    float: left;
    height: 50em;
    background-color: yellow;
}

.global-footer {
    width: 100%;
    float: left;
    min-height: 5em;
    background-color: lightblue;
}

@media (min-width: 30em) {

    .global-header {
        position: fixed;
        top: 0;
        left: 0;
    }

    .main-content {
        height: 100%;
        margin-top: 5em; /* to offset header */
    }

    .global-footer {
        position: fixed;
        bottom: 0;
        left: 0;
    }

} /* ================== */

One possible reason might stem from a parent container housing the CSS property animation. This was the case in my situation.

Don't forget to include the widths of both divs in your CSS!

.header {
    position: fixed;
    top: 0;
    background-color: #f00;
    height: 100px; 
    width: 100%;
}
.footer {
    position: fixed;
    bottom: 0;
    background-color: #f0f;
    height: 120px; 
    width: 100%;
}

Check out the demo here

Ensure to include width and content in your elements. It is also advisable to adjust the padding on the top and bottom of the main element so that the content does not get obscured by the header or footer. Additionally, consider removing the height specifications and allowing the browser to determine the appropriate size based on the content.

Check out this example on JSFiddle

.header{
position: fixed;
background-color: #f00;
height: 100px;
width:100%;
}
.main{
background-color: #ff0;
padding-top: 100px;
padding-bottom: 120px;
}
.footer{
position: fixed;
bottom: 0;
background-color: #f0f;
height: 120px;
width:100%;}

Dealing with a similar problem, I found that the solution was to remove the transform-style: preserve-3d; property from my parent element.