draw diagonal lines in div background with CSS

Solution that automatically scales to dimensions of an element would be usage of CSS3 linear-gradient connected with calc() as shown below. (There were some issues with Chrome in times of v30+ versions that this answer originally described but looks like they got resolved in the meantime and in v90+ it works as expected).

.crossed {
     background: 
         linear-gradient(to top left,
             rgba(0,0,0,0) 0%,
             rgba(0,0,0,0) calc(50% - 0.8px),
             rgba(0,0,0,1) 50%,
             rgba(0,0,0,0) calc(50% + 0.8px),
             rgba(0,0,0,0) 100%),
         linear-gradient(to top right,
             rgba(0,0,0,0) 0%,
             rgba(0,0,0,0) calc(50% - 0.8px),
             rgba(0,0,0,1) 50%,
             rgba(0,0,0,0) calc(50% + 0.8px),
             rgba(0,0,0,0) 100%);
}

<textarea class="crossed"></textarea>

You can do it something like this:

<style>
    .background {
        background-color: #BCBCBC;
        width: 100px;
        height: 50px;
        padding: 0; 
        margin: 0
    }
    .line1 {
        width: 112px;
        height: 47px;
        border-bottom: 1px solid red;
        -webkit-transform:
            translateY(-20px)
            translateX(5px)
            rotate(27deg); 
        position: absolute;
        /* top: -20px; */
    }
    .line2 {
        width: 112px;
        height: 47px;
        border-bottom: 1px solid green;
        -webkit-transform:
            translateY(20px)
            translateX(5px)
            rotate(-26deg);
        position: absolute;
        top: -33px;
        left: -13px;
    }
</style>
<div class="background">
    <div class="line1"></div>
    <div class="line2"></div>
</div>

Here is a jsfiddle.

Improved version of answer for your purpose.

You can use SVG to draw the lines.

.diag {
    background: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' version='1.1' preserveAspectRatio='none' viewBox='0 0 100 100'><path d='M1 0 L0 1 L99 100 L100 99' fill='black' /><path d='M0 99 L99 0 L100 1 L1 100' fill='black' /></svg>");
    background-repeat:no-repeat;
    background-position:center center;
    background-size: 100% 100%, auto;
}

<div class="diag" style="width: 300px; height: 100px;"></div>

Have a play here: http://jsfiddle.net/tyw7vkvm

All other answers to this 3-year old question require CSS3 (or SVG). However, it can also be done with nothing but lame old CSS2:

.crossed {
    position: relative;
    width: 300px;
    height: 300px;
}

.crossed:before {
    content: '';
    position: absolute;
    left: 0;
    right: 0;
    top: 1px;
    bottom: 1px;
    border-width: 149px;
    border-style: solid;
    border-color: black white;
}

.crossed:after {
    content: '';
    position: absolute;
    left: 1px;
    right: 1px;
    top: 0;
    bottom: 0;
    border-width: 149px;
    border-style: solid;
    border-color: white transparent;
}

<div class='crossed'></div>

Explanation, as requested:

Rather than actually drawing diagonal lines, it occurred to me we can instead color the so-called negative space triangles adjacent to where we want to see these lines. The trick I came up with to accomplish this exploits the fact that multi-colored CSS borders are bevelled diagonally:

.borders {
    width: 200px;
    height: 100px;
    background-color: black;
    border-width: 40px;
    border-style: solid;
    border-color: red blue green yellow;
}

<div class='borders'></div>

To make things fit the way we want, we choose an inner rectangle with dimensions 0 and LINE_THICKNESS pixels, and another one with those dimensions reversed:

.r1 { width: 10px;
      height: 0;
      border-width: 40px;
      border-style: solid;
      border-color: red blue;
      margin-bottom: 10px; }
.r2 { width: 0;
      height: 10px;
      border-width: 40px;
      border-style: solid;
      border-color: blue transparent; }

<div class='r1'></div><div class='r2'></div>

Finally, use the :before and :after pseudo-selectors and position relative/absolute as a neat way to insert the borders of both of the above rectangles on top of each other into your HTML element of choice, to produce a diagonal cross. Note that results probably look best with a thin LINE_THICKNESS value, such as 1px.

intrepidis' answer on this page using a background SVG in CSS has the advantage of scaling nicely to any size or aspect ratio, though the SVG uses <path>s with a fill that doesn't scale so well.

I've just updated the SVG code to use <line> instead of <path> and added non-scaling-stroke vector-effect to prevent the strokes scaling with the container:

<svg xmlns='http://www.w3.org/2000/svg' version='1.1' preserveAspectRatio='none' viewBox='0 0 100 100'>
  <line x1='0' y1='0' x2='100' y2='100' stroke='black' vector-effect='non-scaling-stroke'/>
  <line x1='0' y1='100' x2='100' y2='0' stroke='black' vector-effect='non-scaling-stroke'/>
</svg>

Here's that dropped into the CSS from the original answer (with HTML made resizable):

.diag {
  background: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' version='1.1' preserveAspectRatio='none' viewBox='0 0 100 100'><line x1='0' y1='0' x2='100' y2='100' stroke='black' vector-effect='non-scaling-stroke'/><line x1='0' y1='100' x2='100' y2='0' stroke='black' vector-effect='non-scaling-stroke'/></svg>");
  background-repeat: no-repeat;
  background-position: center center;
  background-size: 100% 100%, auto;
}

<div class="diag" style="width: 200px; height: 150px; border: 1px solid; resize: both; overflow: auto"></div>