I'm sorry if my question is stupid, but I can't understand what "a picture is fixed with regard to the viewport" means and how it's connected to the following thing:
Suppose that we have a html element <div class="background"></div>
and some styles
.background {
background: url(some-image.jpg) no-repeat fixed;
height: 500px;
width: 150px; /* suppose that this width is less than a width of the picture */
}
In this case, the background image does not appear. I can't understand why and how it's related to an idea about viewport.