Parallel — Markdown notebook

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Minimal code/text composition with syntax and expiration controls.
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<main>
  <header>
    <b>parallel</b><span>THINK ON ONE SIDE. SEE ON THE OTHER.</span
    ><button id="change" aria-pressed="false">Focus on reading</button>
  </header>
  <section class="document-title">
    <small>PERSONAL NOTEBOOK / ENTRY 004</small>
    <h1>A note about making things</h1>
    <p>A small Markdown subset: headings, paragraphs and bullet lists. Raw HTML stays text.</p>
  </section>
  <section class="split">
    <div class="source">
      <h2>THE DRAFT</h2>
      <label class="sr-only" for="markdown">Markdown source</label
      ><textarea id="markdown" spellcheck="false" maxlength="15000">
# Start before it is perfect

Some ideas only become clear when you give them a little room outside your head.

## A few useful things

- Make the first version small.
- Leave a note for your future self.
- Keep the part that surprised you.

The next step does not have to be the final one.</textarea>
    </div>
    <div class="reading">
      <h2>THE PAGE</h2>
      <article id="rendered"></article>
    </div>
  </section>
  <output data-status aria-live="polite"></output>
  <footer class="demo-note">Fictional brand · local interface demo · no information is sent</footer>
</main>

* {
  box-sizing: border-box;
}
html {
  scrollbar-gutter: stable;
}
body {
  margin: 0;
  font:
    14px/1.5 Arial,
    Helvetica,
    sans-serif;
  background: #fff;
  color: #202622;
}
h1,
h2,
h3,
p,
figure {
  margin: 0;
}
main,
section,
article,
aside,
header,
nav {
  min-width: 0;
}
button,
input,
select,
textarea {
  font: inherit;
  max-width: 100%;
  min-width: 0;
}
button,
a,
input,
select,
summary {
  touch-action: manipulation;
}
button {
  cursor: pointer;
}
a {
  color: inherit;
}
img {
  display: block;
  max-width: 100%;
  object-fit: cover;
}
svg {
  display: block;
  max-width: 100%;
}
button:focus-visible,
a:focus-visible,
input:focus-visible,
select:focus-visible,
textarea:focus-visible,
summary:focus-visible,
[tabindex]:focus-visible {
  outline: 3px solid #2781da;
  outline-offset: 3px;
}
[hidden] {
  display: none !important;
}
[data-status] {
  display: block;
  min-height: 24px;
  font-size: 12px;
  margin-top: 14px;
}
.demo-note {
  font:
    9px/1.6 ui-monospace,
    monospace;
  padding: 16px 0;
  opacity: 0.75;
}
.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  overflow: hidden;
  clip-path: inset(50%);
}
dialog {
  max-width: calc(100vw - 32px);
  width: 480px;
  border: 1px solid #666;
  padding: 25px;
  background: #fff;
  color: #202622;
}
dialog::backdrop {
  background: #0008;
}
dialog h2 {
  margin-bottom: 15px;
}
dialog p {
  margin: 12px 0;
}
dialog button {
  margin-top: 16px;
}
@media (prefers-reduced-motion: reduce) {
  *,
  *:before,
  *:after {
    animation: none !important;
    transition: none !important;
    scroll-behavior: auto !important;
  }
}
body {
  background: #eae9e2;
  color: #4d554b;
}
main {
  max-width: 1150px;
  margin: auto;
  padding: 25px 30px;
}
header {
  display: flex;
  align-items: center;
  gap: 25px;
  border-bottom: 1px solid #bec6b5;
  padding-bottom: 20px;
}
header > b {
  font:
    28px Georgia,
    serif;
  letter-spacing: -1px;
}
header > span {
  font: 6px monospace;
  letter-spacing: 1px;
  color: #99a18c;
}
button {
  margin-left: auto;
  background: transparent;
  border: 1px solid #a2b090;
  padding: 8px 11px;
  color: #738461;
  font-size: 9px;
}
.document-title {
  margin: 25px 0;
}
.document-title > small {
  font: 6px monospace;
  letter-spacing: 1px;
  color: #96a084;
}
h1 {
  font:
    30px Georgia,
    serif;
  letter-spacing: -1px;
  margin: 12px 0;
}
.document-title p {
  font-size: 10px;
  line-height: 1.7;
  color: #98a088;
}
.split {
  display: grid;
  grid-template-columns: 1fr 1fr;
  border: 1px solid #bfcab3;
  background: #f6f7ef;
}
.split > div > h2 {
  font: 7px monospace;
  letter-spacing: 1px;
  color: #98a78a;
  border-bottom: 1px solid #d8dfcd;
  padding: 12px 20px;
}
.source {
  border-right: 1px solid #c8d3bb;
}
textarea {
  display: block;
  width: 100%;
  height: 325px;
  border: 0;
  resize: vertical;
  background: #f0f3e8;
  color: #718365;
  padding: 20px;
  font: 11px/1.8 monospace;
}
#rendered {
  padding: 20px;
}
#rendered h2 {
  font:
    25px/1.2 Georgia,
    serif;
  margin: 5px 0 15px;
}
#rendered h3 {
  font:
    18px Georgia,
    serif;
  margin: 20px 0 12px;
}
#rendered p,
#rendered li {
  font:
    12px/1.85 Georgia,
    serif;
  color: #7b8b6d;
}
#rendered p {
  margin: 10px 0;
}
#rendered ul {
  padding-left: 20px;
}
.focus .split {
  grid-template-columns: 1fr;
  max-width: 650px;
  margin: auto;
}
.focus .source {
  display: none;
}
.focus #rendered {
  padding: 30px;
}
.focus #rendered p,
.focus #rendered li {
  font-size: 17px;
}
@media (max-width: 550px) {
  main {
    padding: 20px;
  }
  header > span {
    display: none;
  }
  h1 {
    font-size: 27px;
  }
  .split {
    grid-template-columns: 1fr;
  }
  .source {
    border-right: 0;
    border-bottom: 1px solid #c8d3bb;
  }
  textarea {
    height: 220px;
    font-size: 10px;
  }
  .focus #rendered {
    padding: 20px;
  }
}

(() => {
  "use strict";
  const $ = (s) => document.querySelector(s),
    $$ = (s) => [...document.querySelectorAll(s)];
  const status = (text) => {
    const el = $("[data-status]");
    if (el) el.textContent = text;
  };
  const escapeText = (s) =>
    String(s).replaceAll("&", "&amp;").replaceAll("<", "&lt;").replaceAll(">", "&gt;");
  function choose(selector, fn) {
    $$(selector).forEach((b) =>
      b.addEventListener("click", () => {
        $$(selector).forEach((x) => x.setAttribute("aria-pressed", String(x === b)));
        fn(b);
      }),
    );
  }
  function filter(selector, items, key) {
    choose(selector, (b) => {
      $$(items).forEach(
        (el) => (el.hidden = b.dataset[key] !== "all" && el.dataset.kind !== b.dataset[key]),
      );
      status($$(items).filter((el) => !el.hidden).length + " results shown.");
    });
  }
  function textList(target, values) {
    $(target).replaceChildren(
      ...values.map((text) => {
        const p = document.createElement("p");
        p.textContent = text;
        return p;
      }),
    );
  }
  function render() {
    const out = $("#rendered");
    out.replaceChildren();
    let list = null;
    for (const line of $("#markdown").value.split("\n")) {
      if (!line.trim()) {
        list = null;
        continue;
      }
      let el;
      if (line.startsWith("- ")) {
        if (!list) {
          list = document.createElement("ul");
          out.append(list);
        }
        el = document.createElement("li");
        el.textContent = line.slice(2);
        list.append(el);
        continue;
      }
      list = null;
      const tag = line.startsWith("## ") ? "h3" : line.startsWith("# ") ? "h2" : "p";
      el = document.createElement(tag);
      el.textContent = tag === "h3" ? line.slice(3) : tag === "h2" ? line.slice(2) : line;
      out.append(el);
    }
  }
  render();
  $("#markdown").addEventListener("input", () => {
    render();
    status("Preview updated as escaped text.");
  });
  $("#change").addEventListener("click", () => {
    const on = document.body.classList.toggle("focus");
    $("#change").setAttribute("aria-pressed", String(on));
    $("#change").textContent = on ? "Back to split view" : "Focus on reading";
    status(on ? "Reading focus enabled." : "Split writing view restored.");
  });
})();