Pure JavaScript: Hide all text on the page but preserve element spacing.

hide-text.js

function textNodesUnder(el){
  var n, a = [];
  var walk = document.createTreeWalker(el, NodeFilter.SHOW_TEXT, null, false);
  while (n = walk.nextNode()) { a.push(n); }
  return a;
}

// Normalize all text nodes (merge adjacent nodes).
var elements = document.getElementsByTagName('*');
for (var i = 0; i < elements.length; i++) {
  elements[i].normalize();
}

elements = textNodesUnder(document.getElementsByTagName('body')[0]);
for (var i = 0; i < elements.length; i++) {
  // Skip empty nodes.
  if (elements[i].nodeValue.trim() == '') { continue; }

  // Create a wrapper span with opacity: 0 and replace the current textNode.
  var wrapperSpan = document.createElement('span');
  wrapperSpan.setAttribute('style', 'opacity: 0');
  wrapperSpan.appendChild(elements[i].cloneNode());
  elements[i].parentNode.replaceChild(wrapperSpan, elements[i]);
}

jquery-hide-text.js

// Or, in jQuery:
$('*').contents().filter(function(i, node) {
  return node.nodeType === 3 && node.textContent.trim() !== '';
}).wrap('<span style="opacity: 0"></span>');