How can I create a text box for a note in markdown?

Question

What I usually do for putting alert box (e.g. Note or Warning) in markdown texts (not only when using pandoc but also every where that markdown is supported) is surrounding the content with two horizontal lines:

---
**NOTE**

It works with almost all markdown flavours (the below blank line matters).

---

which would be something like this:

---

NOTE

It works with all markdown flavours (the below blank line matters).

---

The good thing is that you don't need to worry about which markdown flavour is supported or which extension is installed or enabled.

EDIT: As @filups21 has mentioned in the comments, it seems that a horizontal line is represented by *** in RMarkdown. So, the solution mentioned before does not work with all markdown flavours as it was originally claimed.

With GitHub, I usually insert a blockquote.

> **_NOTE:_**  The note content.

becomes...

NOTE: The note content.

Of course, there is always plain HTML...

The simplest solution I've found to the exact same problem is to use a multiple line table with one row and no header (there is an image in the first column and the text in the second):

----------------------- ------------------------------------
![Tip](images/tip.png)\ Table multiline text bla bla bla bla
                        bla bla bla bla bla bla bla ... the
                        blank line below is important 

----------------------------------------------------------------

Another approach that might work (for PDF) is to use Latex default fbox directive :

 \fbox{My text!}

Or FancyBox module for more advanced features (and better looking boxes) : http://www.ctan.org/tex-archive/macros/latex/contrib/fancybox.

The following methods work on GitHub, on GitLab... and on Stackoverflow, which now uses CommonMark!

> One-Line Box made with Blockquote

One-Line Box made with Blockquote

`One-Line Box made with Backticks`

One-Line Box made with Backticks

```
Box made with Triple Backticks
```

Box made with Triple Backticks  

~ ~ ~
Box made with Triple Tildes _{^{(remove the spaces between the tildes to make this work)}}
~ ~ ~

Box made with Triple Tildes

Box made with Four Spaces at the start of each line:

    “Sometimes we must let go of our pride and do what is requested of us.”
    Padmé Amidala

... or use horizontal lines?

Three dashes (---) make a horizontal line:

---

Note: “ Your focus determines your reality.” – Qui-Gon Jinn.

---

For more configurations, I strongly advise the excellent GitLab Markdown Guide.
You can also check the less detailed GitHub basic formatting syntax.
You can compare Markdown implementations using Babelmark.

Useful hints :

• to force a newline, put two spaces at the end of the line;

• to escape special characters, use \.

Use the admonition extension. For mkdocs, it can be configured in the mkdocs.yml file:

markdown_extensions:
    - admonition

Then insert the note in your md files as follows:

!!! note

     This is a note.

See an example here.