Setting CSS attributes based on the parent node: What you need to know

Here is my CSS code snippet:

.over_center {
position:absolute;
top: 50%;
left:50%;
height: 160px; /* two thirds (2/3) of parent size */
width: 160px; /* two thirds (2/3) of parent size */
margin-top: -80px; /* negative one third of parent size */
margin-left: -80px; /* negative one third of parent size */
line-height: 160px; /* two thirds (2/3) of player size */
text-align: center;
}

What stands out in the code is the use of comments to indicate that the values are relative to the parent size, which is always square (width=height).

While JavaScript can be used to dynamically style these elements, I am intrigued by the possibility of achieving this directly in CSS for a cleaner approach.

Answer №1

Depending on the parent element's positioning, the way absolute positioning works can vary.

If the parent element is set to position: relative, it will serve as the containing block for your element. In this case, using percentages for properties like top and left is appropriate. For example:

height: 66%;
width: 66%;
margin-top: -33%;
margin-left: -33%;

Most percentages, except for height (including margin-top), are relative to the containing block's width. Knowing it's a square simplifies the calculation.

On the other hand, line-height is usually relative to the font size, not the height. If that's a requirement, you'll need to manually set the absolute value. Fortunately, only one value needs updating if the parent's dimensions change.

However, without position: relative on the parent element, you can't use percentages for these properties. In that case, hardcoding the values becomes necessary.

Answer №2

When dealing with positioning elements in CSS, utilizing percentages can be quite helpful:

.over_center {
    position: absolute;
    top: 50%;
    left: 50%;
    height: 66.66%;       /* Percentage of containing block's height */
    width: 66.66%;        /* Percentage of containing block's width */
    margin-top: -33.33%;  /* Percentage of containing block's width */
    margin-left: -33.33%; /* Percentage of containing block's width */
    line-height: 160px;   /* Potential issue here */
    text-align: center;
}

It's important to note that this approach may not be successful if the parent element does not act as the containing block. In such cases, especially when the parent element has position: absolute, the containing block is determined by the closest ancestor with a non-static positioning.

For properties like line-height (and others when the parent is not the containing block), you have a few alternatives:

  • Implement JavaScript
  • Employ Server-side scripting
  • Utilize a CSS preprocessor
  • Experiment with CSS variables. Caution: this feature is still experimental and may have limited browser support.

Answer №3

While CSS doesn't offer an easy solution, you can simplify the process by using a preprocessor like LESS:

@size: 200px;
.centered_element {
    height: @size * 2 / 3;
    width: @size * 2 / 3;
    margin-top: @size / -3;
    margin-left: @size / -3;
    line-height: @size * 2 / 3;
}

This approach allows you to make global changes with just one adjustment.

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