Can you have a 0.5px border on a Retina Display?

Question

On Mobile Safari on an iPhone 4 or iPhone4S, can you have a border of a div that is 0.5px wide?

Answer 1

Apple added support for this in OSX Yosemite and iOS 8.

@media (-webkit-min-device-pixel-ratio: 2){
    div {
        border-width:0.5px;
    }
}

Answer 2

Also I've found this method working (iOS7):

    background: repeat-x top left url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='1' height='1'><rect fill='#ff0000' x='0' y='0' width='1' height='0.5'/></svg>");

Answer 3

I'm not sure what you mean exactly. If you are asking if you can draw a border of 1 px on an iPhone 4 which would be 1/2 the physical size of the border on an iphone3G, then yes. CoreGraphics uses points instead of pixels. As you can see by the following method, you can specify a float:

void CGContextSetLineWidth (
   CGContextRef c,
   CGFloat width
);

Since 1point != 1px, you can specify 1px on an iPhone4 by specifying 0.5point.

Read up on it here

Answer 4

I wrote an overview of different techniques:

Half-pixel border

border: 0.5px solid black;

Cons:

• Works only in Firefox and Webkit Nightly.

border-image

border-width: 1px;
border-image: url(border.gif) 2 repeat;

border.gif is a 6×6 pixel image:

Pros:

• It works!

Cons:

• An external image. It’s only 51 bytes and it can be inlined using Data URI. You’d need to fire up Photoshop (or whatever you use) to change the border color, which isn’t very convenient.

Multiple background images

background:
    linear-gradient(180deg, black, black 50%, transparent 50%) top    left  / 100% 1px no-repeat,
    linear-gradient(90deg,  black, black 50%, transparent 50%) top    right / 1px 100% no-repeat,
    linear-gradient(0,      black, black 50%, transparent 50%) bottom right / 100% 1px no-repeat,
    linear-gradient(-90deg, black, black 50%, transparent 50%) bottom left  / 1px 100% no-repeat;

“How to target physical pixels on retina screens with CSS” describes how to draw a line. Draw 4 lines and we have a border.

Pros:

• No external images.

Cons:

• Cumbersome syntax, although it can be abstracted out with CSS preprocessors.

Scale up and down

Mentioned here already by Priit Pirita.

Answer 5

Yes. Use scale. The style below will give you hairline

 .transform-border-hairline {
     border-bottom: 1px #ff0000 solid;
     transform: scaleY(0.5);
 }

When you need all sides, then the best way is to repliacte the DIV with :after or :before CSS pseudoclass, apply border to that pseudoclass, make it twice the size and then scale it with transform:scale down to half.

pre:before {
    position: absolute;
    top: 0;
    left: 0;
    width: 200%;
    height: 200%;
    border: 1px #999 solid;
    content: "";
    transform: scale(0.5);
    -webkit-transform: scale(0.5);
    transform-origin: 0 0;
    -webkit-transform-origin: 0 0;
}

More specificly (and all those tricks in use too) here http://atirip.com/2013/09/22/yes-we-can-do-fraction-of-a-pixel/

Answer 6

Sharing a SASS/SCSS + Compass mixin I wrote and I've been using:

@mixin hairline($color, $side: 'top') {
  &:before {
    content: ' ';
    display: block;
    @if $side == 'top' {
      width: 100%;
      height: 1px;
    }
    @else if $side == 'bottom' {
      width: 100%;
      height: 1px;
    }
    @else {
      width: 1px;
      height: 100%;
    }
    position: absolute;
    #{$side}: 0;
    background-color: $color;
    @media (-webkit-min-device-pixel-ratio: 2) {
      @if $side == 'top' {
        @include transform(scaleY(0.5));
      } @else if $side == 'bottom' {
        @include transform(scaleY(0.5));
      } @else {
        @include transform(scaleX(0.5));
      }
    }
  }
}

CSS example:

.my-element-where-i-need-a-retina-bottom-border {
  @include hairline(red, 'bottom');
}