Set a Fixed div to 100% width of the parent container

Tackling padding predicaments

Padding patrol with calc()

Parent padding playfully pushes the child to break the 100% width rule because of how the CSS box model functions. Enter the calc() function to clean up padding misdemeanors:

.child {
  width: calc(100% - 20px); /* Parent padded, time to counter */
}
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Margin over padding for element contouring

Margins can safely form bubbles around wrappers without inflating child elements. This fashion leaves child elements unswollen by padding:

.wrap {
  margin: 10px; /* Personal space: 10px. No trespassing. */
}
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Flexible and responsive design strategies

The % game

Percentage values for responsive design mean your layout (and you) stays cool under varying viewport pressures.

.parent {
  width: 80%; /* Let's not occupy everything, shall we? */
}
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Positioning elements with precision

Absolute positioning within a relative parent is your bread-and-butter for fixed elements. But keep an eye out on unwanted overlaps – z-index is your trusty traffic cop:

.child {
  position: absolute;
  z-index: 10; /* Now you see me, now you still do. */
}
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Advanced layout armoury

Choosing the right CSS weapon

Different battleground scenarios require different CSS weapons. Are flex and grid dazzling? Yes. Are they always the answer? Consider deploying traditional methods when you want to K.I.S.S (Keep It Simple, Stupid):

.wrap {
  float: left; /* Float like a butterfly */
  position: relative; /* Not all those who wander are lost */
}
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Pulling out the jQuery artillery

If you're already in the trenches with jQuery, why not call in some fire support to dynamically set your fixed div width:

$(window).on('resize', function() {
  $('.child').width($('.parent').width());
});
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But remember, jQuery is heavy artillery. Don't bring out the tanks when a few sharpshooter rounds of CSS will do.

Cross-browser warfare strategies

Field testing layouts

A CSS' soldier's mantra: Train as you fight. Different browsers might not follow your orders as you'd expect. Tools like JSFiddle simulate these different battlegrounds and allow you to adapt proactively:

[[JSFiddle link here]] - War-game simulation highlighting responsive width fix.

Dealing with browser insubordination

Ensure your CSS troops can handle the occasional insubordination from browsers. Knowing how to standardize element sizing across browsers can earn you the Medal of Honor:

* {
  box-sizing: border-box; /* One box model to rule them all */
}
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