What is OnlineViewer Dev?

OnlineViewer Dev is a free, privacy-first web developer suite offering 7 client-side tools (JSON Viewer, HTML Sandbox, XML Formatter, JavaScript Minifier, CSS Converter, Text Diff Checker, and Markdown Preview) executed entirely inside your browser memory without server processing.

Is my data sent to any backend server?

No. 100% of parsing, formatting, minification, and conversion happens locally in your web browser. Source code, JSON payloads, and text strings are never transmitted over the network.

What developer tools are included in OnlineViewer Dev?

The suite includes JSON Tree & Text Viewer with YAML conversion, HTML Live preview with device viewports, XML Formatter & XML-to-JSON engine, JavaScript Beautifier & Console Sandbox, CSS Formatter & px-to-rem converter, Text Diff Checker with Base64/URL encoders, and Markdown Live preview with Markdown-to-HTML conversion.

Everything runs in your browser

Format, convert and decode developer data.

Free tools for JSON, XML, HTML, CSS, JavaScript, Base64, JWT and Markdown. Paste, pick an action, copy the result. Nothing you paste is uploaded.

Find a tool

Popular: JSON Formatter JSON Viewer Base64 Decode JWT Decoder Diff Checker XML to JSON

All tools

Browse the full list →

JSON

XML

HTML

CSS

JavaScript

Encode & decode

Text & Markdown

Stays on your machine

The tools run as JavaScript in the page. What you paste is never sent to a server.

No sign-up

Free to use, with no account and no paid plan. Open a tool and paste.

Works offline

After your first visit the pages are cached, so they open without a connection.

From the blog

All posts →

GuidesWhy does atob fail with "The string to be decoded is not correctly encoded"?atob throws 'not correctly encoded' when input isn't plain Base64: URL-safe - and _, a data: prefix, extra = padding or a cut-off string. Fixes for each. GuidesIs Base64 encryption?Base64 is not encryption. It is a reversible encoding with no key, so anyone can decode it. Use AES for secrets and Argon2id or bcrypt for passwords. GuidesBase64 vs Base64URL: what is the difference?Base64 vs Base64URL: the URL-safe variant swaps + and / for - and _, and usually drops the = padding. The decoded bytes are the same. Privacy & securityHow to debug REST API responses privatelyHow client-side formatters handle API keys and credentials without sending them anywhere. String manipulationHow to unescape double backslashes in JSON stringsFix double-escaped JSON and see where the extra backslashes come from. JSONHow to format JSON and convert it to YAMLPretty-printing, validation and turning JSON into YAML config.

Questions

Is my code or data uploaded to any server?

No. Formatting, parsing, and minification happen entirely inside your web browser. Your code and payloads are never sent over the network.

Which tool should I open first?

Start with the data type you already have. JSON for payload inspection, HTML for live rendering, Markdown for docs preview, XML for document cleanup, JavaScript or CSS for formatting, and Text for diffs or encoders.

Can I test responsive HTML layouts here?

Yes. You can test your HTML live and toggle mobile, tablet, or desktop viewports in real time.