Practical Guide to Using CSS Position Relative & Absolute

An important concept to understanding relative/absolute positioning is render flow.

The general idea is that HTML elements all take up some space. Your browser’s rendering engine always renders everything in a grid-like fashion, starting at the top-left corner and moving successively towards the bottom-right until it’s done placing all of your HTML content.

If you’ve ever had a slow internet connection, and watched as large stuff on the webpage would push everything rightward and downward, that is essentially “render flow” in action.

You can change this default behavior using CSS position.

CSS position is sometimes considered an advanced skill because it’s not as intuitive as font-size or margin, etc., since it changes the natural “render flow” of the browser.

These are the possible values for CSS position:

.foo {
  position: static;
  /* position: relative;
  position: absolute;
  position: sticky;
  position: fixed; */
}

Today we’re just going to look at position: absolute and position: relative since they’re perhaps the most versatile ones that will get you a lot of mileage once you feel confident with them.

When you make an HTML element position: relative, it’ll remain “in the flow” of the layout but you can move it around!

.green-square {
  position: relative;
  top: 25px;
  left: 25px;
  /* ... */
}

Along with position: relative you’ll usually want to define the top, right, bottom, or left offset.

You can think of “relative” position as being: “relative to where it was initially positioned.” In this case, the green square is now 25px from the left, and 25px from the top of where it was initially going to be.

What’s also worth noting is that its width and height is preserved in the square grid. That means it’s still considered “in the flow” of the layout… it just got kinda nudged.

Absolute positioning is a very powerful CSS rule for moving HTML elements around. Sometimes yielding unexpected results:

.orange-square {
  position: absolute;
  top: 0px;
  left: 0px;
  /* ... */
}

The orange square is actually the 13th of these 25 squares (the one in the middle of the grid), but it looks like it’s the last square! Weird. Using position: absolute takes elements “out of flow” so its grid space gets collapsed.

Yea but why’s it all the way up there?!

Originating coordinates

The orange square gets placed at the 0x, 0y coordinates (eg.: the top-left corner). Just how browser rendering always begins at the top-left corner, position: absolute elements use that as their rendering origin too. You can use top/right/bottom/left properties to offset it from there.

But, you can also give it different originating coordinates…

.grid {
  position: relative;
}
.orange-square {
  position: absolute;
  top: 0px;
  left: 0px;
  /* ... */
}

In the example above, the parent element (div.grid) has the position: relative rule which causes the orange square to take that as its rendering origin.

While this may seem unintuitive behavior, it’s actually intentional! Allowing for this gives you a lot of control over where/how you arrange HTML elements…

When you start using position: relative and position: absolute it opens a new world of design possibilities. You can create layered visual elements, and feel a deep sense of confidence about how browsers will render, and thus place the visual elements that you’ve so meticulously designed.