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Build a lightweight single-file web application with no external dependencies
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| name | tool-web |
| description | Build a lightweight single-file web application with no external dependencies |
| argument-hint | <description of what to build> |
Build a self-contained, single-file .html web application. Everything โ HTML, CSS, JavaScript โ lives in one file. No build step, no frameworks, no external dependencies.
.html file โ all CSS in <style>, all JS in <script>console.logfile:/// where possible (see Compatibility section)Any of these rules may be broken with a good reason and explicit user approval.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Tool Name</title>
<meta name="apple-mobile-web-app-capable" content="yes">
<meta name="apple-mobile-web-app-status-bar-style" content="black-translucent">
<meta name="apple-mobile-web-app-title" content="Tool Name">
<style>
/* CSS here */
</style>
</head>
<body>
<!-- Markup here -->
<script>
/* JS here */
</script>
</body>
</html>
Keep it minimal. No unnecessary meta tags, no favicon link (browsers handle the 404 silently).
The boilerplate above includes the meta tags needed for iOS "Add to Home Screen" to launch as a full-screen standalone app (no Safari chrome).
Safe areas โ opt in only if you really want edge-to-edge. The default boilerplate omits viewport-fit=cover because it has a real downside in regular Safari: the page extends behind the URL bar overlay, which then covers the top of your content (and the soft keyboard accessory covers the bottom). For most tools the default โ Safari constrains the page to its safe area โ is what you want.
If you genuinely want edge-to-edge content (typically only worth it in standalone PWA mode, where Safari's chrome is gone), add viewport-fit=cover back to the viewport meta and use env() to keep content out of the notch / home indicator:
<meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
body {
padding-top: env(safe-area-inset-top);
padding-left: env(safe-area-inset-left);
padding-right: env(safe-area-inset-right);
padding-bottom: env(safe-area-inset-bottom);
}
Or apply selectively to specific containers โ a full-bleed header might want to extend behind the status bar with its own internal padding.
Status bar โ black-translucent makes the status bar overlay the page with white text. This looks best when the page has a dark or colored header. Use default for a standard light status bar, or black for a solid black bar.
Touch behavior โ prevent rubber-band overscroll and accidental text selection in app-like UIs:
html { overscroll-behavior: none; }
body { -webkit-user-select: none; user-select: none; }
/* Re-enable selection on content that should be selectable */
.selectable { -webkit-user-select: text; user-select: text; }
Standalone detection โ detect if running as a home screen app:
const isStandalone = window.navigator.standalone === true
|| window.matchMedia('(display-mode: standalone)').matches;
Note: Add to Home Screen requires the page to be served over HTTPS (or localhost). It will not work from file:/// URLs. If the tool is intended for home screen use, mention this to the user.
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
body { -webkit-font-smoothing: antialiased; }
img, svg { display: block; max-width: 100%; }
input, button, textarea, select { font: inherit; }
No min-height: 100dvh on body. It's a footgun for any UI with a sticky-bottom element: iOS Safari doesn't shrink dvh when the soft keyboard appears, so the body stays full-viewport-tall and your input bar ends up below the keyboard. If you need full-height layout, drive it from visualViewport.height via JS โ see the iOS Safari section.
.template { display: none !important; }
Use system font stacks from modernfontstacks.com. No web fonts, no Google Fonts.
Recommended pairings โ pick a readable stack for body, something with more character for headings:
Body/UI (prioritize readability):
system-ui, sans-serifInter, Roboto, 'Helvetica Neue', 'Arial Nova', 'Nimbus Sans', Arial, sans-serifSeravek, 'Gill Sans Nova', Ubuntu, Calibri, 'DejaVu Sans', source-sans-pro, sans-serifHeadings (more character):
Avenir, Montserrat, Corbel, 'URW Gothic', source-sans-pro, sans-serifOptima, Candara, 'Noto Sans', source-sans-pro, sans-serif'Iowan Old Style', 'Palatino Linotype', 'URW Palladio L', P052, serifCharter, 'Bitstream Charter', 'Sitka Text', Cambria, serifDidot, 'Bodoni MT', 'Noto Serif Display', 'URW Palladio L', P052, Sylfaen, serifCode:
ui-monospace, 'Cascadia Code', 'Source Code Pro', Menlo, Consolas, 'DejaVu Sans Mono', monospaceAll available stacks for reference:
system-ui, sans-serifCharter, 'Bitstream Charter', 'Sitka Text', Cambria, serif'Iowan Old Style', 'Palatino Linotype', 'URW Palladio L', P052, serifSeravek, 'Gill Sans Nova', Ubuntu, Calibri, 'DejaVu Sans', source-sans-pro, sans-serifAvenir, Montserrat, Corbel, 'URW Gothic', source-sans-pro, sans-serifOptima, Candara, 'Noto Sans', source-sans-pro, sans-serifInter, Roboto, 'Helvetica Neue', 'Arial Nova', 'Nimbus Sans', Arial, sans-serif'Nimbus Mono PS', 'Courier New', monospaceui-monospace, 'Cascadia Code', 'Source Code Pro', Menlo, Consolas, 'DejaVu Sans Mono', monospaceBahnschrift, 'DIN Alternate', 'Franklin Gothic Medium', 'Nimbus Sans Narrow', sans-serif-condensed, sans-serifui-rounded, 'Hiragino Maru Gothic ProN', Quicksand, Comfortaa, Manjari, 'Arial Rounded MT', 'Arial Rounded MT Bold', Calibri, source-sans-pro, sans-serifRockwell, 'Rockwell Nova', 'Roboto Slab', 'DejaVu Serif', 'Sitka Small', serifSuperclarendon, 'Bookman Old Style', 'URW Bookman', 'URW Bookman L', 'Georgia Pro', Georgia, serifDidot, 'Bodoni MT', 'Noto Serif Display', 'URW Palladio L', P052, Sylfaen, serif'Segoe Print', 'Bradley Hand', Chilanka, TSCu_Comic, casual, cursiveAs fonts get larger, decrease weight so perceived line thickness stays roughly constant:
h1 { font-size: 2.5rem; font-weight: 300; }
h2 { font-size: 1.75rem; font-weight: 400; }
h3 { font-size: 1.25rem; font-weight: 500; }
body { font-size: 1rem; font-weight: 400; }
small { font-size: 0.875rem; font-weight: 450; }
Adjust to taste โ the principle is visual consistency of stroke width across sizes, not exact numbers.
Mobile-first. Use min-width breakpoints:
.container { padding: 1rem; max-width: 100%; }
@media (min-width: 640px) { .container { padding: 2rem; } }
@media (min-width: 1024px) { .container { max-width: 960px; margin: 0 auto; } }
Use clamp() for fluid sizing: font-size: clamp(1rem, 2.5vw, 1.25rem);
Test at 320px minimum viewport width. No horizontal scrolling at any size.
iOS Safari has several layout quirks that other browsers โ including Chrome on iOS, which uses the same WebKit engine but a different chrome โ don't expose you to. Hit these once and you'll lose an afternoon. Here's the survival kit.
Soft keyboard doesn't shrink 100dvh. On iOS Safari, the dynamic viewport unit ignores the on-screen keyboard. interactive-widget=resizes-content in the viewport meta is silently ignored by WebKit (the console literally says "not recognized"). If your UI has a sticky-bottom element (chat input, action bar, sticky CTA), it'll end up behind the keyboard.
The fix is to mirror visualViewport.height into a CSS variable (or set body.style.height directly) from JS:
function trackViewport() {
const vv = window.visualViewport;
const h = vv ? vv.height : window.innerHeight;
const w = vv ? vv.width : window.innerWidth;
document.documentElement.style.setProperty('--vvh', h + 'px');
document.documentElement.style.setProperty('--vvw', w + 'px');
document.body.style.height = h + 'px';
document.body.style.width = w + 'px';
}
trackViewport();
window.addEventListener('resize', trackViewport);
window.addEventListener('orientationchange', trackViewport);
window.visualViewport?.addEventListener('resize', trackViewport);
window.visualViewport?.addEventListener('scroll', trackViewport);
Setting body.style.height directly (rather than just exposing --vvh for CSS to consume) sidesteps a quirk where iOS Safari sometimes doesn't recompute height: var(--vvh) reliably when the viewport changes. Inline style wins.
Layout viewport can be wider than the visible window. On newer iPhones (17 Pro, etc.) window.innerWidth reports e.g. 377 but the <html> element renders at 402 โ that 25pt gap is content you've rendered off-screen. Same JS as above pins both documentElement.style.width and body.style.width to visualViewport.width, which is the actual visible width.
Native <select> ignores CSS width. iOS pads native dropdowns for touch targets โ set width: 140px and you'll get ~160px in practice, which can blow out a tight header layout. Kill the native chevron with appearance: none and supply your own via a background SVG:
.select {
appearance: none;
-webkit-appearance: none;
background: var(--bg-secondary)
url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 12 12' fill='none' stroke='%23888' stroke-width='1.5' stroke-linecap='round' stroke-linejoin='round'><polyline points='3 5 6 8 9 5'/></svg>")
no-repeat right 8px center;
background-size: 12px 12px;
padding: 8px 26px 8px 10px;
width: 140px; /* now actually 140px */
text-overflow: ellipsis;
overflow: hidden;
white-space: nowrap;
}
Long unbreakable tokens overflow the page. word-wrap: break-word (or its modern alias overflow-wrap: break-word) only breaks if no other break point exists, and Safari's heuristic for "possible break point" is permissive enough that a 30-char user-ID-style string stays intact and pushes the message box past the viewport. Use the more aggressive value:
.msg, .content {
overflow-wrap: anywhere;
}
anywhere lets the browser break mid-token when needed. Safe for all content.
Diagnostic overlay. When you can't get Safari Web Inspector cabled up, an in-page overlay reading window.innerWidth/innerHeight, visualViewport.{width,height,offsetTop}, documentElement.{scrollWidth,offsetWidth}, and body.{scrollWidth,offsetWidth,style.width,style.height} is the fastest way to figure out what's actually happening. Hide it behind ?debug=1 so it's there when you need it:
<div id="dbg" style="position:fixed;top:0;left:0;width:var(--vvw,100%);font:10px ui-monospace,monospace;color:#fff;background:rgba(220,0,0,0.85);padding:2px 6px;white-space:pre-wrap;z-index:99999;display:none"></div>
<script>
if (new URLSearchParams(location.search).has('debug')) {
const el = document.getElementById('dbg');
el.style.display = 'block';
function snap() {
const vv = window.visualViewport;
el.textContent = [
`win ${window.innerWidth}x${window.innerHeight}`,
`vv ${vv ? vv.width.toFixed(0)+'x'+vv.height.toFixed(0) : '-'}`,
`html ow=${document.documentElement.offsetWidth} oh=${document.documentElement.offsetHeight}`,
`body ow=${document.body.offsetWidth} oh=${document.body.offsetHeight}`,
].join('\n');
}
snap();
setInterval(snap, 500);
window.visualViewport?.addEventListener('resize', snap);
}
</script>
#111. Secondary: #555. Tertiary: #999Put user-configurable values (colors, sizes, defaults, thresholds) in a clear config object at the top of <script> with comments explaining each option:
const CONFIG = {
maxItems: 50, // Maximum items to display
refreshMs: 5000, // Auto-refresh interval
defaultTheme: 'light', // 'light' or 'dark'
};
This makes it easy for users to customize behavior without digging through code.
Include these at the top of <script> (after config):
const $ = (sel, ctx = document) => ctx.querySelector(sel);
const $$ = (sel, ctx = document) => [...ctx.querySelectorAll(sel)];
function el(tag, ...args) {
const e = document.createElement(tag);
for (const arg of args) {
if (typeof arg === 'string') e.append(arg);
else if (arg instanceof Node) e.appendChild(arg);
else if (arg && typeof arg === 'object') {
for (const [k, v] of Object.entries(arg)) e.setAttribute(k, v);
}
}
return e;
}
$ and $$ mirror browser devtools conventions. $$ returns a real Array (not NodeList), so .map(), .filter(), .find() all work. The second argument scopes the search:
$('.sidebar') // first match in document
$$('.item') // all matches โ Array
$$('.item').map(i => i.textContent) // chainable
$('.name', card) // scoped to a parent element
$$('.tag', card) // all .tag within card
el() takes a tag name, then any mix of strings (text content), objects (attributes), and Nodes (children):
el('hr')
el('input', { type: 'text', placeholder: 'Search...' })
el('p', 'Hello world')
el('div', { class: 'card' },
el('h2', 'Title'),
el('p', 'Description')
)
el('ul', ...items.map(item => el('li', item.name))) // spread for arrays
el() is deliberately simple โ no event binding, no style objects, no special-casing. Attach listeners on the returned element: myBtn.onclick = handler or myBtn.addEventListener('click', handler).
Templates are real HTML elements hidden by the .template class. Clone them to create instances. This keeps markup in the HTML where it's visible and editable โ no template strings, no innerHTML.
HTML:
<ul id="people">
<li class="template person">
<span class="name"></span> โ <span class="role"></span>
</li>
</ul>
JS:
function stamp(selector, { parent, position = 'append' } = {}) {
const tmpl = $(selector);
const clone = tmpl.cloneNode(true);
clone.classList.remove('template');
const container = parent || tmpl.parentNode;
if (position === 'prepend') container.prepend(clone);
else if (position === 'before') tmpl.before(clone);
else if (position === 'after') tmpl.after(clone);
else container.appendChild(clone);
return clone;
}
const people = {};
function addPerson(id, name, role) {
const p = stamp('.person.template');
$('.name', p).textContent = name;
$('.role', p).textContent = role;
p.dataset.id = id;
people[id] = p;
}
function removePerson(id) {
people[id].remove();
delete people[id];
}
function updatePerson(id, name, role) {
const p = people[id];
$('.name', p).textContent = name;
$('.role', p).textContent = role;
}
Why this pattern works:
$('.name', p) reads naturallystamp() options: parent targets a different container, position controls insertion ('append', 'prepend', 'before', 'after')Encode meaningful UI state in location.hash so the page can be refreshed, deep-linked, and shared:
function getHash() {
return Object.fromEntries(new URLSearchParams(location.hash.slice(1)));
}
function setHash(params) {
location.hash = new URLSearchParams(params).toString();
}
window.addEventListener('hashchange', render);
window.addEventListener('DOMContentLoaded', render);
function render() {
const state = getHash();
// update DOM based on state
}
Flat key=value pairs only. URLSearchParams handles encoding/decoding automatically.
Examples: #view=settings, #q=search+term&page=2, #tab=history&id=42.
Prefer delegating events to a container rather than attaching listeners to every element. This works automatically for stamped elements without re-attaching listeners:
$('#people').addEventListener('click', e => {
const person = e.target.closest('.person');
if (!person) return;
const id = person.dataset.id;
// handle click
});
Use HTML's built-in interactive elements before building custom ones:
<details> / <summary> for collapsible sections<dialog> for modals (with .showModal())<input type="date|time|color|range"> for pickers<progress> and <meter> for progress/gauges<datalist> for autocomplete suggestions<fieldset> / <legend> for grouped form controlsDefault: no external dependencies. The page should require zero network requests beyond itself.
If a dependency is genuinely needed (non-trivial feature unreasonable to reimplement), it must be:
If a dependency is approved, load from one of these:
cdn.jsdelivr.net โ reliable, serves npm packages, supports SRIunpkg.com โ serves npm packages directlycdnjs.cloudflare.com โ Cloudflare-backed, curated setesm.sh โ ESM module CDN, great for modern-browser-only targetsAlways pin the version and use SRI hashes:
<script src="https://cdn.jsdelivr.net/npm/marked@14.1.0/marked.min.js"
integrity="sha384-..." crossorigin="anonymous"></script>
| Library | Size | Purpose | Why acceptable |
|---|---|---|---|
marked | ~40KB | Markdown โ HTML | Complex parser, mature, zero deps |
DOMPurify | ~20KB | HTML sanitization | Security-critical, hard to DIY safely |
highlight.js | ~30KB core | Syntax highlighting | Complex grammars, unreasonable to rewrite |
Papa Parse | ~25KB | CSV parsing | Edge cases (nested quotes, streaming) |
Sortable | ~40KB | Drag-and-drop sorting | Touch support, animations, edge cases |
| Library/Category | Reason |
|---|---|
| React, Vue, Angular, Svelte, htm | Frameworks / framework-like โ defeats the entire purpose |
| jQuery | Unnecessary โ modern DOM APIs cover everything |
| Bootstrap, Tailwind, Bulma | CSS frameworks โ violates no-framework rule |
| Lodash / Underscore | Native JS covers it all: Array.prototype.*, Object.entries, structuredClone, etc. |
| Axios | fetch() is built-in |
| Moment.js | Deprecated, enormous; use Intl.DateTimeFormat or native <input type="date"> |
| D3 (full bundle) | Massive (~240KB); if charting needed, build simple SVG charts by hand or use a tiny focused lib |
| Any library with transitive dependencies | Pulls in a tree of unknowns โ violates standalone rule |
@media (prefers-color-scheme: dark) if appropriate for the toolfile:/// CompatibilityThe page should work when opened directly as a file. This means:
Works from file:///:
location.hash for statelocalStorage / sessionStorage<canvas>crypto, Intl, Web Audio, etc.)Does NOT work from file:///:
fetch() to relative paths โ CORS restriction on file: originsimport โ blocked by CORS in some browsersSharedArrayBuffer / COOP/COEP headersIf the tool requires HTTP-only features, tell the user and suggest:
python3 -m http.server 8000
Test with rodney, a Chrome automation CLI:
# Start visible browser
rodney start --show
# Open the page (file:// or http://)
rodney open file:///path/to/tool.html
# Verify basics
rodney title
rodney waitidle
rodney exists ".expected-element"
rodney visible "#main"
# Test interactions
rodney click ".button"
rodney input "#search" "test query"
rodney text ".result"
# Run JS assertions
rodney js "document.querySelectorAll('.item').length"
rodney assert "document.title !== ''"
# Check accessibility tree
rodney ax-tree
rodney stop
Write a brief test sequence and run it to verify the tool works after building it.
rodney drives headless Chrome. It's fast, scriptable, and great for cross-page logic โ but it does not reproduce Safari-specific bugs (URL bar overlay, native <select> sizing, soft-keyboard layout, word-break heuristics). If your tool is going to be used on iPhone, layer in the higher tiers below.
Tier 1 โ Chrome via rodney. Smoke tests, interactions, JS assertions. What's shown above.
Tier 2 โ WebKit engine via Playwright. Closer to Safari than Chrome โ catches word-break, viewport behavior, sticky positioning differences. Doesn't simulate Safari's browser chrome (URL bar, accessory bar, native control rendering).
npm i -D playwright
npx playwright install webkit
import { webkit, devices } from 'playwright';
const browser = await webkit.launch();
const ctx = await browser.newContext(devices['iPhone 15 Pro']);
const page = await ctx.newPage();
await page.goto('http://localhost:8000/tool.html');
// page.evaluate / page.screenshot / page.click / ...
Tier 3 โ actual Safari via iOS Simulator. Real Safari on simulated iPhone. Catches everything Tier 2 misses โ URL bar overlay, native control sizing, keyboard accessory overlap, viewport meta tag interpretation.
# One-time: open Xcode.app once to bootstrap CoreSimulator. simctl will hang
# until you've done this. Then in Xcode โ Settings โ Platforms, install an iOS
# runtime (~7GB) if none is listed.
# List available iPhone devices
xcrun simctl list devices available | grep iPhone
# Boot one
xcrun simctl boot 'iPhone 17 Pro'
# Open Safari to a URL
xcrun simctl openurl booted "http://localhost:8000/tool.html"
# Take a screenshot
xcrun simctl io booted screenshot /tmp/sim.png
# Tap at coords (in screen points)
xcrun simctl io booted tap 195 400
The screenshots from xcrun simctl io ... screenshot are pixel-accurate to real iPhones. For interactive debugging, plug a real iPhone in or use the simulator and attach macOS Safari's Web Inspector via the Develop menu (Mac Safari โ Settings โ Advanced โ Show Develop menu, then Develop โ [your device] โ page URL).
Auth-gated pages. If your tool requires HTTP Basic / Bearer auth, neither Playwright nor simctl openurl accepts credentials in the URL reliably (modern Safari strips them). Workarounds:
browser.newContext({ httpCredentials: {...}, extraHTTPHeaders: { Authorization: '...' } })Pick the lowest tier that catches the bug you're chasing. Most CSS/layout changes only need Tier 1โ2; iOS-Safari-specific issues need Tier 3.