What HTML tags are allowed on Stack Exchange sites?

Question

The Stack Overflow site engine, as you know, uses Markdown for questions and answers. Per the Markdown specification, you are allowed to freely intermix HTML and Markdown tags.

Not all HTML tags are allowed, as that would be an XSS paradise. Which HTML tags have been whitelisted and are allowed to be used on Stack Exchange sites?

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Answer 1

Allowed HTML Tags

The Stack Exchange engine allows only the following safe, whitelisted subset of HTML tags.

Press your middle mouse button and wave your mouse left or right to scroll the table, or use your laptop's trackpad (if applicable):

Tag	Title and Description	Preview	Markdown Equivalent
<a>	Anchor Used to create hyperlinks.	StackExchange	[StackExchange](https://meta.stackexchange.com)
<b>	Bring Attention To Used to draw the reader's attention to the contents, which are not otherwise granted special importance.	StackExchange Renders the same as <strong>.	None, **this** syntax produces <strong>
<blockquote>	Block Quotation Indicates that the enclosed text is an extended quotation.	StackEchange	> StackExchange
<code>	Inline Code Indicates that the text is a short fragment of computer code.	StackExchange	`StackExchange`
<del>	Deleted Text A range of text that has been deleted from a document.	StackExchange	None on SE
<dd>	Description Details Provides the description, definition, or value for the preceding term (<dt>) in a description list (<dl>).	StackExchange THIS IS A <dd>	None
<dl>	Description List A description list.	Term Definition Another term Another definition	None
<dt>	Description Term A term in a description or definition list.	THIS IS A <dt> StackExchange	None
<em>	Emphasis Marks text that has stress emphasis.	StackExchange Renders same as <i>.	*StackExchange* _StackExchange_
<h1> <h2> <h3>	Section Heading Only <h1>–<h3>, not later	StackExchange StackExchange StackExchange	Header 1 ======== Header 2 -------- # Header 1 # ## Header 2 ## ### Header 3 ###
<i>	Idiomatic Text A range of text that is set off from the normal text for some reason.	StackExchange Renders same as <em>.	None, *this* syntax produces <em>
<img>	Image Embed Embeds an image into the document.		
<kbd>	Keyboard Input A span of inline text denoting textual user input from a keyboard, voice input, or any other text entry device.	StackExchange	None
<li>	Used to represent an item in a list.	These are list items	- List item + List item * List item 1. List item
<ol>	Ordered List An ordered list of items—typically rendered as a numbered list.	Step one Step two Step three	1. Step one 2. Step two 3. Step three
<p>	Paragraph.	StackExchange No visual changes, used to create explicit paragraphs	StackExchange
<pre>	Preformatted Text Preformatted text which is to be presented exactly as written.	StackExchange	StackExchange ``` StackExchange ```
<s>	Renders text with a strikethrough, or a line through it.	StackExchange	None
<sup>	Superscript Inline text which should be displayed as superscript.	TextStackExchange Because <small> is not on this list, many users use <sup> as an alternative.	None
<sub>	Subscript Inline text which should be displayed as subscript.	TextStackExchange	None
<strong>	Strong Importance Indicates that the contents have strong importance, seriousness, or urgency. Renders in bold type.	StackExchange	**StackExchange** __StackExchange__
<strike>	Strikethrough is deprecated, use <del> or <s> instead.	N/A	None
<ul>	Unordered List	An item Another item	- An item - Another item - A third item + An item + Another item + A third item * An item * Another item * A third item
<br>	Line Break Produces a line break in text (carriage-return)	StackExchange StackExchange	Two spaces at the end of the first line: StackExchange   StackExchange
<hr>	Defines a thematic change in the content, usually via a horizontal line.		Must be an empty line before and after the dashes: ---
<!-- […] -->	Defines a comment, or text that will not be rendered in the post.		No true Markdown way

Note that since we allow the above HTML tags, to have tags show up as text you need to escape the < character. You can do this by marking it as code (with indentation or backticks as is done below) or by using the HTML entity <.

Any tags not in the above list will be stripped out of the post. Do note that the sanitizer that strips out tags will recognize any text between < and > signs as HTML, and so any such text will be stripped out of the post; to prevent this, encode those signs as < and >. This does also mean that HTML comments cannot contain the > sign.

Allowed attributes

Attributes are only allowed on <img> and <a>. Other tags with attributes will be stripped.

Allowed attributes for <img>

The following attributes are allowed on the <img> tag, in the order shown below. Note that the mobile theme enforces a maximum width of 90%, so specifying a height might not scale the image proportionally on the mobile sites.

Attributes must be specified in this order. You may omit attributes you do not want to specify, but specifying a later attribute before an earlier one will strip the tag.

1. src=""
2. width="" (1–999; plain number without any extensions like px)
3. height="" (1–999; plain number without any extensions like px; pay attention to order warning above)
4. alt=""
5. title=""

Allowed attributes for <a>

Once again, attributes specified in reverse order to what is shown here will cause the tag to be stripped:

1. href=""
   This attribute can only hold proper URL links, such as https://stackexchange.com/users/, or relative or protocol-relative URL links such as /q/3122 or //stackexchange.com/users/. They cannot hold other types of links, such as relative fragments (e.g. #3122), or protocols other than HTTP or HTTPS (e.g. javascript, ftp, or mailto). Invalid hrefs will strip the entire <a> tag.
2. title=""

Important Notes

• HTML tags unlisted above are stripped from the output. They may render in the client preview, but they will always be removed on the server.
• The tags must be entered exactly as shown in the list. Exactly one space must be present between attributes with no spaces between the attribute name, = sign, and double quotes "".
  • Valid: <a href="...">
  • Invalid: <a href="...">
  • Invalid: <a href= "...">
• While it is possible to make tables using Markdown syntax, we do not support the HTML <table> element.