Hacky PHP to produce a "blank" web page which somehow has content when viewed in Firefox. Sample page at https://danq.me/wp-content/no-code-webpage/, explanation at https://danq.me/nocode

_no_code_page_.php

<?php
// half-hearted CSS minification
$css = preg_replace(
  array('/\s*(\w)\s*{\s*/','/\s*(\S*:)(\s*)([^;]*)(\s|\n)*;(\n|\s)*/','/\n/','/\s*}\s*/'), 
  array('$1{ ','$1$3;',"",'} '),
  file_get_contents('linked.css')
);

// embed as a data: uri
$base64css = rtrim(strtr(base64_encode($css), '+/', '-_'), '=');
header("Link: <data:text/css;charset=utf-8;base64,${base64css}>; rel=\"stylesheet\"");

// no page content ;-)
?>

linked.css

body {
  background-attachment: fixed;
  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='100%25' height='100%25' viewBox='0 0 1600 800'%3E%3Cg %3E%3Cpath fill='%230e009d' d='M486 705.8c-109.3-21.8-223.4-32.2-335.3-19.4C99.5 692.1 49 703 0 719.8V800h843.8c-115.9-33.2-230.8-68.1-347.6-92.2C492.8 707.1 489.4 706.5 486 705.8z'/%3E%3Cpath fill='%230700a4' d='M1600 0H0v719.8c49-16.8 99.5-27.8 150.7-33.5c111.9-12.7 226-2.4 335.3 19.4c3.4 0.7 6.8 1.4 10.2 2c116.8 24 231.7 59 347.6 92.2H1600V0z'/%3E%3Cpath fill='%230001ac' d='M478.4 581c3.2 0.8 6.4 1.7 9.5 2.5c196.2 52.5 388.7 133.5 593.5 176.6c174.2 36.6 349.5 29.2 518.6-10.2V0H0v574.9c52.3-17.6 106.5-27.7 161.1-30.9C268.4 537.4 375.7 554.2 478.4 581z'/%3E%3Cpath fill='%23000ab3' d='M0 0v429.4c55.6-18.4 113.5-27.3 171.4-27.7c102.8-0.8 203.2 22.7 299.3 54.5c3 1 5.9 2 8.9 3c183.6 62 365.7 146.1 562.4 192.1c186.7 43.7 376.3 34.4 557.9-12.6V0H0z'/%3E%3Cpath fill='%230014BA' d='M181.8 259.4c98.2 6 191.9 35.2 281.3 72.1c2.8 1.1 5.5 2.3 8.3 3.4c171 71.6 342.7 158.5 531.3 207.7c198.8 51.8 403.4 40.8 597.3-14.8V0H0v283.2C59 263.6 120.6 255.7 181.8 259.4z'/%3E%3Cpath fill='%230024c8' d='M1600 0H0v136.3c62.3-20.9 127.7-27.5 192.2-19.2c93.6 12.1 180.5 47.7 263.3 89.6c2.6 1.3 5.1 2.6 7.7 3.9c158.4 81.1 319.7 170.9 500.3 223.2c210.5 61 430.8 49 636.6-16.6V0z'/%3E%3Cpath fill='%230036d6' d='M454.9 86.3C600.7 177 751.6 269.3 924.1 325c208.6 67.4 431.3 60.8 637.9-5.3c12.8-4.1 25.4-8.4 38.1-12.9V0H288.1c56 21.3 108.7 50.6 159.7 82C450.2 83.4 452.5 84.9 454.9 86.3z'/%3E%3Cpath fill='%23004ae4' d='M1600 0H498c118.1 85.8 243.5 164.5 386.8 216.2c191.8 69.2 400 74.7 595 21.1c40.8-11.2 81.1-25.2 120.3-41.7V0z'/%3E%3Cpath fill='%230061f2' d='M1397.5 154.8c47.2-10.6 93.6-25.3 138.6-43.8c21.7-8.9 43-18.8 63.9-29.5V0H643.4c62.9 41.7 129.7 78.2 202.1 107.4C1020.4 178.1 1214.2 196.1 1397.5 154.8z'/%3E%3Cpath fill='%230179FF' d='M1315.3 72.4c75.3-12.6 148.9-37.1 216.8-72.4h-723C966.8 71 1144.7 101 1315.3 72.4z'/%3E%3C/g%3E%3C/svg%3E");
  background-size: cover;
  color: white;
  margin: 0;
}

html::before, html::after, body::before, body::after {
  animation: appear 0.6s cubic-bezier(0.230, 1.000, 0.320, 1.000) both;
  color: white;
  display: block;
  font-family: sans-serif;
}

html::before {
  animation-delay: 0.2s;
  border-bottom: 4px dotted #f0b466;
  content: '🤣 The Page With No Code 💻';
  font-size: 48px;
  font-weight: bold;
  margin: 24px 12px;
  padding-bottom: 24px;
  text-align: center;
}

body::before {
  animation-delay: 0.5s;
  content: "It all started when I saw You Don't Need HTML ( no-ht.ml ) by Terence Eden ( shkspr.mobi/blog/2022/12/you-dont-need-html ). The approach he'd used worked, certainly, but it wasn't any more convoluted than just serving a Unicode plain text file as an index document. Plus: what was with that hacky doctype header? Was it possible, I wondered, to come up with anything... even worse. This web page answers that question with a resounding: yes!";
  font-size: 18px;
  line-height: 1.35;
  margin: 24px 12px;
  text-align: justify;
}

body::after {
  content: "This web page has NO CODE WHATSOEVER*. Use 'View Source' in your browser to see for yourself, or try downloading a copy with curl, wget, or what-have-you. It doesn't work in most browsers - I guess that the developers behind Chrome and Safari don't see the value of this particular kind of hackery - but it works in Firefox and maybe others too.";  animation-delay: 0.8s;
  font-size: 18px;
  line-height: 1.35;
  margin: 24px 12px;
  text-align: justify;
}

html::after {
  animation-delay: 1.1s;
  border-top: 4px dotted #f0b466;
  color: #ddd;
  content: "* Obviously there's some code somewhere. Have a look yourself, first, too see if you can find where it comes from. Then, for an explanation of what you're seeing, go to: danq.me/nocode (sorry that the links aren't clickable).";
  font-size: 14px;
  line-height: 1.2;
  margin: 24px 12px;
  padding-top: 24px;
}

@keyframes appear {
  0% {
    filter: blur(40px);
    opacity: 0;
    transform-origin: 0% 50%;
    transform: translateX(1000px) scaleX(2.5) scaleY(0.2);
  }
  100% {
    filter: blur(0);
    opacity: 1;
    transform-origin: 50% 50%;
    transform: translateX(0) scaleY(1) scaleX(1);
  }
}