Mobile Previewer — Test HTML on mobile in seconds
HTML Source — Mobile Previewer
About this HTML Mobile Previewer
The Mobile Previewer on alltoolsonline.org is built for developers, designers and no-code makers
who want a fast and realistic way to see how their pages behave on different screen sizes. Instead of
opening multiple browser windows or connecting a physical device, you can paste your full HTML
(including <!doctype html>, <html> and <head>)
into the editor and immediately view synchronized previews for mobile, tablet and desktop.
Each preview pane uses a sandboxed iframe with configurable width and height. You get a set of popular presets for common devices, but you can also type your own custom dimensions when you need something more specific, such as a particular Android phone or a landscape tablet mockup. The tool automatically scales the preview to fit the card while still respecting the underlying viewport dimensions, so you can focus on layout and breakpoints instead of constant scrolling.
The workflow is intentionally simple. Start by pasting or typing your HTML in the editor above the
previews. The tool listens to your changes and pushes them into all three iframes using
srcdoc, which means you always see the latest version without refreshing the page.
Whenever you want a fresh baseline, use the “Sample” button to load a mobile-first demo layout that
already includes a proper viewport meta tag and some responsive blocks.
Privacy is a key design principle for this Mobile Previewer. Your HTML is processed entirely inside your browser and is never uploaded to any remote server. This makes the tool safe to use with in-progress client work, internal prototypes or test snippets that include sensitive content. Once you leave or refresh the page, the browser discards the current source automatically.
For quick handoffs or debugging, you can export what you see in the editor. The “Open in new tab”
button writes the current HTML into a blank tab so you can test it as a standalone page, run browser
dev tools or save it locally. The “Download” button generates a preview.html file on
the fly, which is convenient when you want to archive a snapshot, share a minimal reproduction
with another developer, or attach a static example to a support ticket.
The Mobile Previewer is helpful in many scenarios: checking whether your navigation collapses correctly on narrow devices, validating font sizes and line lengths for long-form content, testing hero sections and CTAs on mobile, or verifying how embedded components behave around critical breakpoints. Because everything is driven by plain HTML, you can use it with static mockups, templating output, or even HTML exported from your CMS or email builder.
To get the most realistic results, always include a viewport meta tag such as
<meta name="viewport" content="width=device-width,initial-scale=1"> inside your
<head>. This tells the browser to render the page using the device width and
prevents desktop-style zoomed layouts on small screens. From there you can iterate on typography,
spacing and component behavior while watching the three panes side by side.
Overall, this tool aims to be a lightweight companion to your normal development environment: no accounts, no project setup, and no dependencies beyond a modern browser. Paste HTML, adjust sizes, and ship interfaces that feel good on mobile, tablet and desktop without leaving your browser tab.
Mobile Previewer FAQ
Does my HTML leave my browser?
No. The HTML Mobile Previewer is a completely client-side tool. Your source code is written into sandboxed iframes directly in your browser and is not sent to any external server or stored anywhere on alltoolsonline.org. When you close or refresh the page, your HTML disappears with the session.
Can I test JavaScript inside the preview?
Yes. This HTML Mobile Previewer iframes are created with sandbox="allow-scripts allow-same-origin", which
allows inline scripts to execute inside the preview while keeping them isolated from the parent
page. This is useful for testing interactive components, simple widgets or layout scripts, as long
as they do not rely on browser features blocked by the sandbox.
How do I get accurate mobile and tablet behaviour?
Make sure your HTML includes a viewport meta tag, for example:
<meta name="viewport" content="width=device-width,initial-scale=1">.
Without it, most mobile browsers emulate a large desktop viewport and zoom out, which can make
your design appear smaller or misaligned. With the viewport meta tag in place, the widths you set
in each pane will map much more closely to real devices.
Can I change the device sizes or enter my own dimensions?
Yes. Each pane has a dropdown of popular presets plus two numeric fields for width and height. You can either select a preset or type your own values, then click “Apply”. The tool keeps the configured size for that pane and automatically rescales the preview to fit inside the card.
How do the “Open in new tab” and “Download” buttons work?
“Open in new tab” writes the current HTML from the editor directly into a blank browser tab so
that you can inspect it as a standalone page, use dev tools, or save it manually. “Download”
generates a temporary preview.html file from the same source and triggers a download
in your browser. Both actions operate locally and do not upload your content to a server.
Is this tool suitable for production testing?
The previewer is ideal for quick checks, debugging and visual QA across multiple viewport sizes. For full production testing you should still verify on real devices, device farms or advanced responsive testing suites. Treat this tool as a fast companion for iteration, prototypes and layout validation during development.