Pixel Border and Percentage width in Proportion

Question

I think I might already know the answer to this one but I need a sanity check!

Say I have

#gridtest{
width:590px;
}

I could change the width to a percentage by using RESULT=TARGET/CONTEXT. In this case the context is a container with a max-width set to 1000px so I can do this:

#gridtestpercent{
width:59%; /*590/1000*/
}

If I were to shrink the window down the div would always be in the proportion to the its container. But what if I wanted to do

#gridtest{
width:570px;
border:10px solid red;
}

I can work the width out based on the target now being 570 but as the window is shrunk the proportions all go out of sync.

#gridtestpercentnoborder{
width:57%; /*570/1000*/
border:10px solid red;
}

I can't use percentage border. I don't want to use JS to keep checking the context and I can't use the CSS3 box-border declaration yet.

If I wanted to use the technique described in responsive web design by Ethan Marcotte where everything shrinks in relation to each other would I be out of luck if using a border?

Cheers!

Answer 1

The accepted answer is not correct. You actually have 2 options:

Use the box-sizing property, so all the paddings and borders are considered part of the size:

.column {
    width: 16%;
    float: left;
    margin: 0 2% 0 2%;
    background: #03a8d2;
    border: 2px solid black;
    padding: 15px;
    font-size: 13px;

    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
}

Or, use the outline property instead of the border property. You will still have problems with the paddings, but it's easier to add. Example:

.column {
    width: 16%;
    float: left;
    margin: 0 2% 0 2%;
    background: #03a8d2;

    outline: 2px solid black;
}

Full explanation: http://designshack.net/articles/css/beating-borders-the-bane-of-responsive-layout/

Answer 2

You could use CSS3 calc() function,

.selector{
  border: 5px solid black;
  width: -moz-calc(50% - 10px);
  width: -webkit-calc(50% - 10px);
  width: calc(50% - 10px);
}

SASS mixin

@mixin calc($property, $expression) {
  #{$property}: -moz-calc(#{$expression});
  #{$property}: -webkit-calc(#{$expression});
  #{$property}: calc(#{$expression});
}
article {
  border: 1px solid red;
  @include calc( width, '100% - 2px')
}

Answer 3

You could use an inset box-shadow instead of a border:

box-shadow: 0px 0px 0px 10px red inset;

Just pad the inside of the container to compensate.

Edit: I write "pad" but of course if you use padding it'll throw off the box dimensions. Margin the content inside instead.

Answer 4

Unfortunately, yes, you're out of luck. One hacky way to get around this problem is with a wrapper div that you use to create your border. So the outside div would be 57% (in your example) with a background that is the color of your desired border. Then, the inner div would have a width of 96% or so (play with the exact number to find a border that is appropriate for your design).

Answer 5

If you want to stay semantic you can use div { box-sizing:border-box; } or some absolutely positioned :after elements. See the post How do I add 1px border to a div whose width is a percentage?

Answer 6

In CSS3 you can also use the new box-sizing property to include the pixel and padding count into the width of the element:

box-sizing: border-box;