Markdown Viewer / Browser Extension

Install: Chrome / Firefox / Edge / Opera / Brave / Chromium / Vivaldi

Features

• Secure by design
• Render local and remote file URLs
• Granular access to remote origins
• Multiple markdown parsers
• Full control over the compiler options (marked, remark)
• 30+ Themes (cleanrmd, GitHub)
• GitHub Flavored Markdown (GFM)
• Auto reload on file change
• Syntax highlighted code blocks (prism)
• Table of Contents (ToC)
• MathJax formulas (mathjax)
• Mermaid diagrams (mermaid)
• Convert emoji shortnames (icons provided free by EmojiOne)
• Remember scroll position
• Markdown Content-Type detection
• Configurable Markdown file path detection
• Settings synchronization
• Raw and rendered markdown views
• Free and Open Source

Table of Contents

• After Install
• Compiler Options
• Content Options
• Themes
• Manage Origins
• Markdown Detection
• Syntax Examples

After Install

Local Files

1. Navigate to chrome://extensions
2. Locate the Markdown Viewer extension and click on the Details button
3. Make sure that the Allow access to file URLs switch is turned on

Remote Files

1. Click on the Markdown Viewer icon and select Advanced Options
2. Add the origins that you want enabled for the Markdown Viewer extension

---

Compiler Options

Markdown Viewer can be used with any markdown parser/compiler. Currently the following compilers are supported: marked, remark, showdown, markdown-it, remarkable, commonmark, and officially it ships with Marked and Remark:

Marked

Option	Default	Description
breaks	false	Enable GFM line breaks. This option requires the gfm option to be true.
gfm	true	Enable GFM GitHub Flavored Markdown.
pedantic	false	Conform to obscure parts of markdown.pl as much as possible. Don't fix any of the original markdown bugs or poor behavior.
sanitize	false	Sanitize the output. Ignore any HTML that has been input.
smartypants	false	Use "smart" typographic punctuation for things like quotes and dashes.

Remark

Option	Default	Description
breaks	false	Enable GFM line breaks. This option requires the gfm option to be true.
gfm	true	Enable GFM GitHub Flavored Markdown.
sanitize	false	Sanitize the output. Ignore any HTML that has been input.

For any other supported compiler that you may want to use, follow the Manual Install steps.

---

Content Options

Option	Default	Description
autoreload	false	Auto reload on file change
toc	false	Generate Table of Contents
syntax	true	Syntax highlighted fenced code blocks
mathjax	false	Render MathJax formulas
mermaid	false	Render Mermaid diagrams
emoji	false	Convert emoji :shortnames: into EmojiOne images
scroll	true	Remember scroll position

Autoreload

When enabled the extension will make a GET request every second to markdown files hosted on:

• file:/// URLs
• any host that resolves to localhost IPv4 127.0.0.1 or IPv6 ::1

Emoji

Convert emoji :shortnames: into EmojiOne images:

• Emoji shortnames like: :sparkles: will be converted to ✨ using EmojiOne images.
• Currently unicode symbols like ✨ and ASCII emoji like :D are not supported.

MathJax

The following MathJax delimiters are supported:

• inline math: \(math\) and $math$
• display math: \[math\] and $$math$$

The following rules apply to your markdown content when MathJax is enabled:

• Regular dollar sign $ in text that is not part of a math formula should be escaped: \$
• Regular markdown escaping for parentheses: \( and \), and brackets: \[ and \] is not supported. MathJax will convert anything between these delimiters to math formulas, unless they are wrapped in backticks: `\(` or fenced code blocks.

Mermaid

Render Mermaid diagrams wrapped in mmd or mermaid fenced code blocks:

```mmd
sequenceDiagram
```

Scroll

Remember scroll position:

• When enabled, the scroll option remembers the current scroll position and scrolls back to it after page reload.
• When disabled, the scroll option will either scroll to the top of the document or to a certain header (anchor) if a hash URL fragment was present.

Syntax

Syntax highlighting for fenced code blocks:

```js
var hello = 'hi'
```

The full list of enabled languages and their names can be found here.

ToC

Generates Table of Contents (ToC) based on the headers found in the markdown document.

---

Themes

All themes support the following width options:

• auto - automatically adjust the content width based on the screen size
• full - 100% screen width
• wide - fixed at 1400px
• large - fixed at 1200px
• medium - fixed at 992px
• small - fixed at 768px
• tiny - fixed at 576px

The auto option on the github and github-dark themes has a fixed width with a surrounding border identical to a rendered README.md file for a repository on github.com

---

Manage Origins

Access to local file URLs can be enabled by using the Allow access to file URLs option for the extension.

Access to remote origins, however, have to be enabled explicitly.

Add Origin

Here is how you can enable the extension on the https://raw.githubusercontent.com origin:

The origin consists of a protocol and a domain part. The protocol can be either https, http, or a * to match both https and http.

Allow All Origins

In case you really want to you can enable the extension for all origins:

Alternatively you can use the Allow All button.

Take a look at the Path Matching Priority section below on how the Markdown content can be included for or excluded from rendering when access to all origins is enabled!

Remove Origin

Click on the REMOVE button for the origin that you want to remove. This removes the permission itself so that the extension no longer has access to that origin.

Refresh Origin

The extension synchronizes your preferences across all of your devices using Google Sync. The list of your allowed origins is being synced too. However, the actual permissions that you grant using the Chrome's consent popup cannot be synced.

In case you have added a new origin on some of your devices you will have to explicitly allow it on your other devices. In this case an additional refresh label will be shown on each origin that needs to be refreshed. This label will be present only on those devices and origins that needs to be refreshed, meaning currently the extension does not have access to those origins. Expanding the origin will reveal an additional REFRESH button:

---

Markdown Detection

Markdown Viewer can only access explicitly allowed remote origins and the file origin, if enabled. However, not all content served on those origins will be a Markdown file.

Header Detection

When this option is enabled the extension will check for the existence of the text/markdown and text/x-markdown content-type header before trying to match the path:

Path Matching

If the header detection is disabled or a proper content-type header is missing, the extension will check if the URL is ending with a markdown file extension.

The default regular expression is: \.(?:markdown|mdown|mkdn|md|mkd|mdwn|mdtxt|mdtext|text)(?:#.*|\?.*)?$

It is a simple regular expression that matches URLs ending with:

• markdown file extension: \.(?:markdown|mdown|mkdn|md|mkd|mdwn|mdtxt|mdtext|text)
• and optionally a hash or a querystring after that: (?:#.*|\?.*)?

The ?: used in (?:match) stands for non-capturing group and it's there for performance reasons.

You can modify the path matching regular expression for each enabled origin individually. The settings are being updated as you type.

Path Matching Priority

The enabled origins are matched from most to least specific:

1. https://raw.githubusercontent.com or http://raw.githubusercontent.com
2. *://raw.githubusercontent.com
3. *://*

Only the first matching origin is picked and then its Path Matching RegExp is used to match the entire URL. If it fails then no other attempts are being made to match the URL.

It is recommended to explicitly allow only the origins that you want the extension to have access to.

Custom Path

• *://website.com - specific origin with custom Path Matching RegExp:
  \/some\/custom\/path\/to\/serve\/markdown\/$

• *://* - match all origins using the default Path Matching RegExp:
  \.(?:markdown|mdown|mkdn|md|mkd|mdwn|mdtxt|mdtext|text)(?:#.*|\?.*)?$

In case we want to match a custom path we have to add a more specific origin and set its Path Matching RegExp accordingly.

Exclude Origin

• *://github.com - specific origin with invalid Path Matching RegExp:
  something impossible to match !!!

• *://* - match all origins using the default Path Matching RegExp:
  \.(?:markdown|mdown|mkdn|md|mkd|mdwn|mdtxt|mdtext|text)(?:#.*|\?.*)?$

Some origins may serve rendered HTML content on URLs ending with markdown file extension:

GitHub:

• https://github.com/simov/markdown-syntax/blob/main/README.md - HTML
• https://raw.githubusercontent.com/simov/markdown-syntax/main/README.md - Markdown

In this case we want to exclude the github.com origin altogether and for that we have to add a more specific origin and set its Path Matching RegExp to something that's impossible to match.

Exclude Path

• *://bitbucket.org - specific origin with custom Path Matching RegExp:
  .*\/raw\/.*\.(?:markdown|mdown|mkdn|md|mkd|mdwn|mdtxt|mdtext|text)(?:#.*|\?.*)?$

• *://gitlab.com - specific origin with custom Path Matching RegExp:
  .*\/raw\/.*\.(?:markdown|mdown|mkdn|md|mkd|mdwn|mdtxt|mdtext|text)(?:#.*|\?.*)?$

• *://* - match all origins using the default Path Matching RegExp:
  \.(?:markdown|mdown|mkdn|md|mkd|mdwn|mdtxt|mdtext|text)(?:#.*|\?.*)?$

Some origins may serve rendered HTML content on URLs ending with markdown file extension:

BitBucket:

• https://bitbucket.org/simovelichkov/markdown-syntax/src/main/README.md - HTML
• https://bitbucket.org/simovelichkov/markdown-syntax/raw/main/README.md - Markdown

GitLab:

• https://gitlab.com/simovelichkov/markdown-syntax/blob/main/README.md - HTML
• https://gitlab.com/simovelichkov/markdown-syntax/raw/main/README.md - Markdown

In this case we want to match Markdown files only when their path contains the raw word, and for that we have to add a more specific origin and update its Path Matching RegExp accordingly.

---

Syntax Examples

A separate repository containing examples about the Markdown syntax and all features available in Markdown Viewer can be found on GitHub, GitLab and BitBucket:

• elements.md - quick overview of the Markdown syntax and all features available in Markdown Viewer
• syntax.md - extensive list of Markdown syntax examples with different combinations and edge cases
• prism.md - syntax highlighting examples, for the full list of currently supported languages take a look at this file
• mermaid.md - different types of Mermaid diagrams
• mathjax.md - MathJax examples and support documentation

Allow the appropriate remote origin and update its path matching regexp if needed:

• raw.githubusercontent.com using the default Path Matching RegExp:
  \.(?:markdown|mdown|mkdn|md|mkd|mdwn|mdtxt|mdtext|text)(?:#.*|\?.*)?$

• bitbucket.org only paths containing the raw word and ending with the default Path Matching RegExp:
  .*\/raw\/.*\.(?:markdown|mdown|mkdn|md|mkd|mdwn|mdtxt|mdtext|text)(?:#.*|\?.*)?$

• gitlab.com only paths containing the raw word and ending with the default Path Matching RegExp:
  .*\/raw\/.*\.(?:markdown|mdown|mkdn|md|mkd|mdwn|mdtxt|mdtext|text)(?:#.*|\?.*)?$

Alternatively, pull any of those repositories and access them on the file:/// origin locally.

---

Manual Install

The following instructions applies for: Chrome, Edge, Opera, Brave, Chromium and Vivaldi.

Note that in any of the following cases you won't receive any future updates automatically!

Load packed .crx

1. Go to releases and pick a release that you want to install
2. Download the markdown-viewer.crx file
3. Navigate to chrome://extensions
4. Make sure that the Developer mode switch is enabled
5. Drag and drop the markdown-viewer.crx file into the chrome://extensions page

Load unpacked .zip

1. Go to releases and pick a release that you want to install
2. Download the markdown-viewer.zip file and extract it
3. Navigate to chrome://extensions
4. Make sure that the Developer mode switch is enabled
5. Click on the Load unpacked button and select the extracted directory

Build

1. Clone this repository
2. Execute sh build/package.sh chrome (append compilers to include all available compilers)
3. Navigate to chrome://extensions
4. Make sure that the Developer mode switch is enabled
5. Click on the Load unpacked button and select the cloned directory

Manifest v2

1. Clone the mv2 or compilers-mv2 branch (Markdown Viewer v4.0)
2. Navigate to chrome://extensions
3. Make sure that the Developer mode switch is enabled
4. Click on the Load unpacked button and select the cloned directory

---

License

The MIT License (MIT)

Copyright (c) 2013-present, Simeon Velichkov simeonvelichkov@gmail.com (https://github.com/simov/markdown-viewer)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.