With GitHub, I usually insert a blockquote.

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

becomes...

NOTE: The note content.

Of course, there is always plain HTML...

I usually insert a blockquote and add a Unicode character(memo📝 which is(U+1F4DD)) inside it.

📝 ...

Emoji

Of course, if you do not like 📝 you can search you like. I am sure there will be one in it is your satisfaction!

• find more emoji: https://emojipedia.org/

  just search you like icon and copy-paste then done(since it is a character, so it suitable for every device)

• find Code

  If you don't like copy paste and want to type yourself, you can consider searching the Unicode.

  

p.s. You can also pay attention to the emoji version (usually it is the same as the Unicode version), and more icons may appear in the future to your satisfaction.

• Unicode Version 16.0
• unicode/Public
  • unicode/Public/emoji
    • unicode/Public/emoji/latest/
      • UCD/latest/ucd/EmojiSources.txt
      • emoji/latest/emoji-sequences.txt
      • emoji/latest/emoji-zwj-sequences.txt
      • 🤩 emoji/latest/emoji-test.txt

Optional Reading

The following content is optional to read

• GitHub Alerts
• Neovim ga
• Nerd Fonts

GitHub Alerts

If you're using it for markdown files on GitHub, these work quite well:

> [!NOTE]
> Useful information that users should know, even when skimming content.

> [!TIP]
> Helpful advice for doing things better or more easily.

> [!IMPORTANT]
> Key information users need to know to achieve their goal.

> [!WARNING]
> Urgent info that needs immediate user attention to avoid problems.

> [!CAUTION]
> Advises about risks or negative outcomes of certain actions.

reference:

• basic-writing-and-formatting-syntax#alerts
• discussions/16925

Neovim ga

If you want to find Unicode code points and you're using the neovim,

You can simply use ga in normal mode to see decimal, hexadecimal, and octal results, saving you from having to look up Unicode code points.

📝U+1f4dd

Nerd Fonts

If you're looking for a font with many icons, consider: Nerd Fonts

https://github.com/ryanoasis/nerd-fonts/releases/download/v3.3.0/FiraCode.zip

For example, using: FiraCodeNerdFontMono-Retina.ttf

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 \.

As of Sept 2021 :
Here's one way of building a text box in markdown using html div tag and class 'warning'. It works beautifully in Jupyter Notebook and Typora. You can modify the background and font colors.

<div class="warning" style='padding:0.1em; background-color:#E9D8FD; color:#69337A'>
<span>
<p style='margin-top:1em; text-align:center'>
<b>On the importance of sentence length</b></p>
<p style='margin-left:1em;'>
This sentence has five words. Here are five more words. Five-word sentences are fine. But several together bocome monotonous. Listen to what is happening. The writing is getting boring. The sound of it drones. It's like a stuck record. The ear demands some variety.<br><br>
    Now listen. I vary the sentence length, and I create music. Music. The writing sings. It has a pleasent rhythm, a lilt, a harmony. I use short sentences. And I use sentences of medium length. And sometimes when I am certain the reader is rested, I will engage him with a sentence of considerable length, a sentence that burns with energy and builds with all the impetus of a crescendo, the roll of the drums, the crash of the cymbals -- sounds that say listen to this, it is important.
</p>
<p style='margin-bottom:1em; margin-right:1em; text-align:right; font-family:Georgia'> <b>- Gary Provost</b> <i>(100 Ways to Improve Your Writing, 1985)</i>
</p></span>
</div>

OR (adding a bit more to the design - rounded corners and border)

<div class="warning" style='background-color:#E9D8FD; color: #69337A; border-left: solid #805AD5 4px; border-radius: 4px; padding:0.7em;'>
<span>
<p style='margin-top:1em; text-align:center'>
<b>On the importance of sentence length</b></p>
<p style='margin-left:1em;'>
This sentence has five words. Here are five more words. Five-word sentences are fine. But several together bocome monotonous. Listen to what is happening. The writing is getting boring. The sound of it drones. It's like a stuck record. The ear demands some variety.<br><br>
    Now listen. I vary the sentence length, and I create music. Music. The writing sings. It has a pleasent rhythm, a lilt, a harmony. I use short sentences. And I use sentences of medium length. And sometimes when I am certain the reader is rested, I will engage him with a sentence of considerable length, a sentence that burns with energy and builds with all the impetus of a crescendo, the roll of the drums, the crash of the cymbals -- sounds that say listen to this, it is important.
</p>
<p style='margin-bottom:1em; margin-right:1em; text-align:right; font-family:Georgia'> <b>- Gary Provost</b> <i>(100 Ways to Improve Your Writing, 1985)</i>
</p></span>
</div>

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.

I didn't see the below format listed here yet. It works both in github and obsidian. Looks also pretty =)

> [!EXAMPLE]
> this is an example

> [!NOTE]
> This is a note

Example where it works in github: https://github.com/tasmota/docs-7.1/blob/master/Commands.md

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.

Here's a simple latex-based example.

---
header-includes:
    - \usepackage[most]{tcolorbox}
    - \definecolor{light-yellow}{rgb}{1, 0.95, 0.7}
    - \newtcolorbox{myquote}{colback=light-yellow,grow to right by=-10mm,grow to left by=-10mm, boxrule=0pt,boxsep=0pt,breakable}
    - \newcommand{\todo}[1]{\begin{myquote} \textbf{TODO:} \emph{#1} \end{myquote}}
---

blah blah

\todo{something}

blah

which results in:

Unfortunately because this is latex, you can no-longer include markdown inside the TODO box (which is not a huge problem, usually), and it won't work when converting to formats other than PDF (e.g. html).

Have you tried using double tabs? To make a box:

Start on a fresh line
Hit tab twice, type up the content
Your content should appear in a box

It works for me in a regular Rmarkdown document with html output. The double-tabbed portion should appear in a rounded rectangular light grey box.

Similar to Etienne's solution, a simple table formats nicely:

| | |
|-|-|
|`NOTE` | This is something I want you to notice. It has a lot of text, and I want that text to wrap within a cell to the right of the `NOTE`, instead of under it.|

Another alternative (which comes with more emphasis), is to make the content the header of a body-less table:

|`NOTE` | This is something I want you to notice. It has a lot of text, and I want that text to wrap within a cell to the right of the `NOTE`, instead of under it.|
|-|-|

Finally, you can include a horizontal line (thematic break) to create a closed box (although the line style is a little different than the header line in the table):

| | |
|-|-|
|`NOTE` | This is something I want you to notice. It has a lot of text, and I want that text to wrap within a cell to the right of the `NOTE`, instead of under it.|

---

Note the empty line after the text.

You may also use https://www.npmjs.com/package/markdown-it-container

::: warning
*here be dragons*
:::

Will then render as:

<div class="warning">
   <em>here be dragons</em>
</div>

Another solution is to use CSS adjacency and use h4 (or higher):

#### note

This is the note content

h4 {
  display: none; /* hide */
}

h4 + p {
  /* style the note however you want */
}

single line

> hello
> world

hello world

multi line

method a

> hello
> 
> world (new line)

hello

world (new line)

method b

> ```shell
> echo hello
> echo world
> ```

echo hello
echo world

Here is a haskell filter for warning and tips.

Useful for latex and PDF output.

#!/usr/bin/env runhaskell
{-# LANGUAGE OverloadedStrings #-}
import Text.Pandoc.JSON
import Data.Text

latex::Format
latex = Format "latex"

highlight :: Block -> IO Block
highlight cb@(Div (id, (cls:_), _) (contents:_)) =
   case (unpack cls) of "warn" -> return $ Div ("", [], []) ((RawBlock latex "\\begin{tcolorbox}[colframe=yellow!90!white, colback=yellow!20!white]Warning: ") : contents : (RawBlock latex "\\end{tcolorbox}") : [])
                        "tips" -> return $ Div ("", [], []) ((RawBlock latex "\\begin{tcolorbox}[colframe=blue!20!white, colback=blue!10!white]Tips: ") : contents : (RawBlock latex "\\end{tcolorbox}") : [])
                        _ -> return cb

highlight x = return x

main :: IO ()
main = toJSONFilter highlight

and a header-included file H.tex

\usepackage{tcolorbox}

markdown file example.md

## How to write warning in pandoc

::: warn
deprecated, do not use.
:::

::: tips
usefull tips for writing markdown
:::

compile

pandoc example.md --filter f.hs -o book.pdf -s -H H.tex

Here is HTML filter

#!/usr/bin/env runhaskell
{-# LANGUAGE OverloadedStrings #-}
import Text.Pandoc.JSON
import Data.Text

html::Format
html = Format "html"

highlight :: Block -> IO Block
highlight cb@(Div (id, (cls:_), _) (contents:_)) =
   case (unpack cls) of "warn" -> return $ Div ("", [], []) ((RawBlock html "<div class=\"warn\">⚠warning:") : contents : (RawBlock html "</div>") : [])
                        "tips" -> return $ Div ("", [], []) ((RawBlock html "<div class=\"tips\">📝Tips:") : contents : (RawBlock html "</div>") : [])
                        _ -> return cb

highlight x = return x

main :: IO ()
main = toJSONFilter highlight

header-include file H.html

<style>
div.warn {
    background-color: yellow;
    font-family: monospace;
    border-radius: 5pt;
}
div.tips {
    background-color: lightblue;
    font-family: monospace;
    border-radius: 5pt;
}
</style> 

> __Note__:

That will show note word with icon check result here => Note in md file

I also use the 'table' method from general Markdown language.
Just using the header field only. I feel it's the simplest way to go.

| **Note** : `Some stuff goes here` |
| ----------------------------------|

Looks like this :