H1

H2

H3

H4

H5

H6

Alternatively, for H1 and H2, an underline-ish style:

Alt-H1

Alt-H2

Emphasis, aka italics, with asterisks or underscores.

Strong emphasis, aka bold, with asterisks or underscores.

Combined emphasis with asterisks and underscores.

Strikethrough uses two tildes. Scratch this.

1. First ordered list item
2. Another item

⋅⋅* Unordered sub-list.

1. Actual numbers don't matter, just that it's a number ⋅⋅1. Ordered sub-list
2. And another item.

⋅⋅⋅You can have properly indented paragraphs within list items. Notice the blank line above, and the leading spaces (at least one, but we'll use three here to also align the raw Markdown).

⋅⋅⋅To have a line break without a paragraph, you will need to use two trailing spaces.⋅⋅ ⋅⋅⋅Note that this line is separate, but within the same paragraph.⋅⋅ ⋅⋅⋅(This is contrary to the typical GFM line break behaviour, where trailing spaces are not required.)

• Unordered list can use asterisks

• Or minuses

• Or pluses

I'm an inline-style link

I'm an inline-style link with title

I'm a reference-style link

I'm a relative reference to a repository file

You can use numbers for reference-style link definitions

Or leave it empty and use the link text itself.

Some text to show that the reference links can follow later.

Here's our logo (hover to see the title text):

Inline-style:

Reference-style:

var s = "JavaScript syntax highlighting";
alert(s);

s = "Python syntax highlighting"
print s

No language indicated, so no syntax highlighting. 
But let's throw in a <b>tag</b>.

Here is a simple footnote¹.

A footnote can also have multiple lines².

You can also use words, to fit your writing style more closely³.

My reference.[^6] Every new line should be prefixed with 2 spaces. [^5] This allows you to have a footnote with multiple lines. Named footnotes will still render with numbers instead of the text but allow easier identification and linking.
This footnote also has been made with a different syntax using 4 spaces for new lines.³

Colons can be used to align columns.

There must be at least 3 dashes separating each header cell. The outer pipes (|) are optional, and you don't need to make the raw Markdown line up prettily. You can also use inline Markdown.

renders

Blockquotes are very handy in email to emulate reply text. This line is part of the same quote.

Quote break.

This is a very long line that will still be quoted properly when it wraps. Oh boy let's keep writing to make sure this is long enough to actually wrap for everyone. Oh, you can put Markdown into a blockquote.

Definition list

Is something people use sometimes.

Markdown in HTML

Does not work very well. Use HTML tags.

Three or more...

Hyphens

Asterisks

Underscores

Here's a line for us to start with.

This line is separated from the one above by two newlines, so it will be a separate paragraph.

This line is also a separate paragraph, but... This line is only separated by a single newline, so it's a separate line in the same paragraph.

Checkout my React component:

Footnotes Example

Here’s a note¹.

[!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.