There is a kind of quiet power in simplicity. The kind that doesn't announce itself with flashy UI, subscription popups, or a dozen toolbar buttons you'll never touch. It just works — instantly, elegantly, and without asking anything of you. That is the power of Markdown. And that is exactly what MarkViewer gives you.
In a world bloated with writing tools that demand accounts, licenses, and loading screens, Markdown remains the great equalizer — a syntax so clean you can read it raw, yet so expressive it powers documentation for some of the world's largest software projects, billion-dollar blogs, and the entire GitHub ecosystem.
What Is Markdown, Really?
Markdown was invented by John Gruber in 2004 with a single radical idea: writing for the web shouldn't require knowing HTML. You shouldn't need to type <h1> to make a heading. A # should do the job. You shouldn't wrap text in <strong> to bold it. Two asterisks on each side is enough.
Two decades later, that idea has become the backbone of the modern web's writing layer. GitHub README files, Stack Overflow answers, Reddit posts, Notion pages, VS Code documentation, Discord messages — Markdown is everywhere.
"Markdown is not just a syntax. It's a philosophy: focus on content, not formatting. Think, don't click."
The Case for Plain Text Writing
The Syntax That Disappears
The hallmark of great design is that it becomes invisible. Markdown achieves this in writing. Look at this snippet:
# My Project
## Overview
This tool does **one thing** and does it *brilliantly*.
## Features
- Real-time preview
- Zero configuration
- Works everywhere
> "The best tool is the one that stays out of your way."
No menus. No mouse. No distraction. Just thought translated directly into structured, beautiful text. When you paste this into a Markdown renderer, it becomes a polished, readable document instantly. That's the magic — the syntax never gets in the way of the thinking.
Why Every Developer, Writer & Creator Needs a Fast Viewer
Here's the dirty secret of Markdown: you can write it anywhere, but previewing it used to be a pain. You'd need an IDE, a plugin, or a clunky online tool that takes five seconds to load and then asks you to sign up for a newsletter.
This is exactly the gap that MarkViewer fills. Built by developer V. Senthil, MarkViewer is a real-time Markdown editor and previewer that lives entirely in your browser — no installs, no accounts, no nonsense.
You open it. You type. It renders. That's it.
The Philosophy Behind It
Every great tool begins with a frustration. Senthil's frustration was the same one every developer has had: needing a quick Markdown preview and losing minutes to setup friction. MarkViewer is the antidote — a tool that respects your time and trusts your intelligence.
What started as a focused viewer has evolved into a complete Markdown IDE — with workspace management, a built-in AI assistant, multi-file tabs, a command palette, and more. Yet it never lost its soul: show me what my Markdown looks like, right now.
🛠️ Built-In Power Features
Everything You Need.
Nothing You Don't.
MarkViewer isn't just a preview tool — it's a full-featured Markdown workstation packed with capabilities that rival desktop apps. Here's what's built in:
📂 Workspace Management — Your Files, In the Browser
MarkViewer features a full workspace file system powered by the Origin Private File System (OPFS) with a localStorage fallback. Create multiple workspaces, organize files into folders, and switch between them instantly — all persisted directly in your browser with zero cloud dependency.
The left sidebar gives you a VS Code-style file tree with pinned files, folder expand/collapse, inline rename, context menus, and full-text search across every file in your workspace. Bulk-select files for mass operations. Import/export entire workspaces as JSON backups.
// OPFS-powered persistent storage
Sidebar → File tree with folders, pinned section, search
Workspaces → Create, switch, delete named workspaces
File Tabs → Multi-file editing with drag-to-reorder
Unsaved Dot → Amber indicator on tabs with pending changes
Auto-Save → 30s interval saves to OPFS + localStorage
Bulk Ops → Select multiple files → delete / move
Import/Export → Backup workspaces as JSON
Toggle → ⌘B to show/hide sidebar
🤖 Built-In AI Assistant
MarkViewer ships with a floating AI widget that works directly inside your editor. Press Ctrl+. to summon it. Select text and the AI operates on your selection; leave nothing selected and it works on the entire document.
Three AI providers are supported out of the box: OpenAI (GPT-4o, GPT-4, etc.), Google Gemini (models fetched dynamically from the API), and Ollama (local models auto-detected from your running instance). Responses stream in real-time with Markdown rendering, and you can insert or replace text directly from the widget.
// Three providers, one seamless interface
OpenAI → GPT-4o, GPT-4, GPT-3.5 — streaming responses
Gemini → Dynamic model list via x-goog-api-key
Ollama → Auto-detect local models from /api/tags
// Built-in actions
Summarize · Rewrite · Explain · Fix Grammar
Expand · Simplify · Translate · Custom prompt
// Context-aware
Select text → AI works on selection
No selection → AI works on entire document
⌘ Command Palette & Keyboard-First Design
Press Ctrl+P or ⌘K to open the command palette — a fuzzy-search menu that gives instant access to every command, view toggle, file action, and setting. Navigate to any line with :42 syntax. Every feature in MarkViewer has a keyboard shortcut — from toggling dark mode (⌘⇧K) to opening the AI widget (Ctrl+.) to saving (⌘S).
🔗 Public Share URLs with Expiry
Generate a shareable public URL for any document — your entire Markdown content gets LZ-compressed and embedded directly into the URL. No server, no database, no account needed. Paste the link in any browser and it renders instantly.
Choose a validity: Permanent, 1 Hour, 1 Day, 1 Week, or 30 Days. Expired links are automatically rejected with a clear toast notification. The URL stays in the address bar so you can bookmark and reuse it.
// Your content gets compressed into the URL
https://v-senthil.github.io/mark-viwer/#doc=N4IgDgTg...
// Open it anywhere — incognito, different browser, phone
// The content is IN the URL. No backend required.
// Choose link validity:
Permanent · 1 Hour · 1 Day · 1 Week · 30 Days
🌿 PlantUML Diagrams
Write PlantUML directly in fenced code blocks using plantuml, planttext, or puml as the language tag. Diagrams render live as SVG via the PlantUML server — class diagrams, sequence diagrams, activity diagrams, and more.
```plantuml
@startuml
actor User
participant "Editor" as E
participant "Preview" as P
User -> E: Types Markdown
E -> P: Renders HTML
P --> User: Shows output
@enduml
```
📊 Mermaid Diagrams
Create flowcharts, sequence diagrams, Gantt charts, pie charts, and more using Mermaid syntax — rendered live in the preview pane with full SVG output.
🧮 KaTeX Math Equations
Inline math with $E=mc^2$ and display blocks with $$...$$. Full LaTeX math rendering powered by KaTeX — perfect for technical documents, academic notes, and scientific writing.
🎨
Themes & Fonts
Multiple editor themes (GitHub, Dracula, Nord) and preview themes (GitHub, Notion, Minimal). Choose between Sans Serif, Serif, or Monospace fonts. Customize font sizes and line height to your liking.
🌙
Dark & Light Mode
Full dark mode support across the entire UI — editor, preview, panels, and toolbar. Toggle instantly with a click or Ctrl+Shift+K.
🔗
Public Share URLs
Share any document via URL with configurable expiry — permanent, 1 hour, 1 day, 1 week, or 30 days. Content is compressed into the URL itself. Zero backend.
📐
PlantUML & Mermaid
Render PlantUML class/sequence/activity diagrams and Mermaid flowcharts, Gantt charts, and more — all live in the preview pane.
⌨️
Vim Mode
Full Vim keybindings in the editor for power users. Toggle it on from the View menu or Settings panel — modal editing in the browser.
🧘
Focus & Zen Modes
Focus Mode hides toolbar and statusbar. Zen Mode goes full-screen with a centered editor. Typewriter Mode keeps your cursor vertically centered as you type.
🔄
Sync Scroll
Auto-scroll keeps editor and preview perfectly synchronized. Scroll one panel, the other follows. Toggle it from the toolbar or settings.
📑
Presentation Mode
Turn any Markdown document into a full-screen presentation instantly. Perfect for quick demos, meetings, and walkthroughs.
🛠️
Rich Toolbar
One-click insert for bold, italic, headings, links, images, tables, code blocks, task lists, Mermaid diagrams, PlantUML, and KaTeX math.
📥
Export & Download
Download as .md, export as standalone HTML, or print to PDF. Drag-and-drop .md files to open them. Paste images directly from clipboard.
📚
TOC & Recent Docs
Auto-generated Table of Contents from your headings with click-to-navigate. Recent Documents panel keeps your last 10 documents a click away.
💾
Autosave & Syntax
30-second autosave to OPFS and localStorage. Full syntax highlighting for 100+ languages via highlight.js. Footnote support with hover tooltips.
📂
Workspace Sidebar
VS Code-style file tree with workspace selector, folder navigation, pinned files, search, inline rename, and bulk operations — powered by OPFS.
🗂️
Multi-File Tabs
Open multiple files in draggable tabs with unsaved-change indicators, auto-save on switch, and one-click close. Your tab order is persisted across sessions.
🤖
AI Assistant Widget
Floating AI panel with OpenAI, Gemini & Ollama support. Summarize, rewrite, explain, or generate content — works on selection or entire document.
⌘
Command Palette
Ctrl+P instant fuzzy-search over every command, file, and setting. Navigate to any line with :42 syntax. Keyboard-first productivity.
📊
Analytics Dashboard
Live document stats — word count, character count, reading time, heading structure, and readability metrics in a dedicated panel.
Markdown's Superpower: It Ages Like Fine Wine
Software tools come and go. Editors rise to dominance and fade into obsolescence. But a Markdown file you wrote in 2010 opens perfectly today in any editor, any viewer, any system. That's the promise of plain text — radical longevity.
Compare that to a Word document from 2003. A Google Doc from a deprecated format. A Notion export that doesn't quite look right elsewhere. Markdown outlasts them all because it carries no proprietary baggage. It's just text, and text is forever.
"A Markdown file you write today will be perfectly readable in 2050. Your Word document? Good luck."
Plain Text Permanence
Who Is MarkViewer For?
Developers who want to quickly sanity-check a README before pushing to GitHub. Technical writers who draft documentation and need instant visual feedback. AI builders authoring system prompts, skills, and agent instructions in Markdown. Students writing notes and wanting a clean output. Bloggers who write in Markdown before pasting into their CMS. Anyone who's ever typed two asterisks and wondered "wait, is this going to render?"
In other words, MarkViewer is for people who value their time and appreciate tools that don't get in the way.
The Quiet Revolution You Should Join
We've grown so accustomed to complexity that simplicity feels radical. A tool that does exactly one thing and does it perfectly is almost subversive. MarkViewer is part of a quiet revolution in software: purposeful tools built by developers who are scratching their own itch, sharing it openly, and proving that the best software doesn't always come from a VC-backed startup.
It comes from someone who sat down, saw a gap, and built the thing they wished existed.
That deserves recognition. That deserves your vote. That deserves to be shared with every developer, writer, and creator in your network.
🤖 Markdown × Artificial Intelligence
The Secret Fuel Behind AI Agents & Skills
You've heard about LLMs. You've used ChatGPT, Claude, Gemini. But here's what most people don't realize: Markdown is the invisible lingua franca of the entire AI ecosystem. Nearly every system prompt, every AI skill definition, every agent instruction set — it's all written in Markdown. And MarkViewer is the fastest way to see it rendered, clearly.
"When you talk to an AI agent, you're often talking to a machine reading beautifully structured Markdown. The format shapes the mind."
Prompt Engineering Fundamentals
How LLMs Consume Markdown
Large Language Models like GPT-4, Claude, and Gemini are trained on billions of pages of text — and a staggering proportion of that text is Markdown. GitHub repositories, developer docs, technical wikis, README files — Markdown is the native language of the data that trained modern AI. This means that when you write well-structured Markdown, AI models understand it better, parse it more accurately, and respond to it with greater precision.
This isn't a coincidence. Markdown's clean hierarchy — headings, lists, code blocks, bold emphasis — maps almost perfectly to how language models internally represent structure. It's the format LLMs were born to read.
# System Prompt: Customer Support Agent
## Role
You are a helpful, friendly support agent for MarkViewer.
## Instructions
- Always greet the user by name if known
- Keep responses concise and clear
- If unsure, say: "Let me find out for you"
## Tone
> Warm, professional, never robotic.
## Out of Scope
Redirect billing issues to support@example.com
See that? That's a real AI system prompt. It's pure Markdown. And when an AI agent reads it, it understands the hierarchy, the emphasis, the constraints — just like a human reader would. Markdown makes AI agents smarter by giving them structure they can reason about.
AI Skills Are Written in Markdown
In modern AI frameworks — from LangChain to AutoGen to Claude's tool-use system — "skills" are structured capability definitions that tell AI agents what they can do and how to do it. These skill documents are almost universally authored in Markdown. Headers define the skill name. Lists define steps. Code blocks define examples. The entire cognitive architecture of an AI agent lives inside structured Markdown files.
When you author an AI skill, you're essentially writing a Markdown document that becomes the agent's mind for that task. The clarity, structure, and precision of your Markdown directly determines how well your AI agent performs. Garbage Markdown = garbage agent behavior. Crystal-clear Markdown = a powerful, predictable AI skill.
# Skill: Web Research Agent
## Description
Search the web and summarize findings into structured reports.
## Input Parameters
| Parameter | Type | Required | Description |
|-----------|--------|----------|----------------------|
| query | string | yes | Search topic |
| depth | number | no | Pages to scan (1-10) |
## Steps
1. Parse the **query** for key entities
2. Run search with structured operators
3. Summarize each result in <100 words
4. Return a ranked Markdown report
## Output Format
## Results
### [Source Title]
Summary text here...
Markdown Supercharges AI Agent Workflows
Multi-agent systems — where AI agents plan, delegate, and execute tasks — rely heavily on structured communication between agents. How do agents communicate their findings, pass instructions, or report results? In Markdown. A planning agent writes a Markdown task list. An execution agent reads it, acts on each item, and returns a Markdown report. The orchestrator parses that report and decides what's next.
Markdown is the protocol of the agentic web. It's not just a human writing tool — it's the message format that lets autonomous AI systems coordinate, reason, and build on each other's work with clarity and precision.
🧠
LLM Prompt Engineering
System prompts written in clean, structured Markdown produce dramatically better AI outputs. Headers, lists, and emphasis help models parse context with higher accuracy.
🤖
AI Agent Instructions
Every AI agent has an instruction set — and it's almost always Markdown. Role definitions, constraints, tool descriptions, and escalation rules all live in structured `.md` files.
⚙️
Skill & Tool Definitions
AI skills in LangChain, AutoGen, Claude, and similar frameworks use Markdown to define capabilities, parameters, and expected outputs — turning text into agent superpowers.
🔗
Agent-to-Agent Communication
In multi-agent pipelines, Markdown is the handoff format. Agents pass structured reports, plans, and findings between each other using Markdown's natural hierarchy.
📊
RAG Knowledge Bases
Retrieval-Augmented Generation systems store knowledge as Markdown documents. Well-formatted Markdown improves chunk quality, retrieval accuracy, and final answer quality.
🎯
Few-Shot Examples
Teaching AI models new behaviors with few-shot examples? Those examples are formatted in Markdown. The cleaner the examples, the faster the model adapts.
"MarkViewer isn't just for humans previewing docs. It's for AI builders previewing the prompts, skills, and instructions that power their agents — before they go live."
The AI Builder's Workflow
This is where MarkViewer becomes not just useful, but essential for the AI age. When you're writing a system prompt for your agent, you want to see it rendered — to catch missing structure, broken lists, or formatting errors before your AI acts on them. When you're authoring a skill definition or a RAG knowledge document, a clean live preview catches problems that a raw text editor never would.
Paste your prompt. See it rendered. Fix it. Ship it. MarkViewer is your AI prompt debugging tool.