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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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A couple of people have sort of hinted at this, but there is limited markup available for comments. There is more information on this here - meta.stackexchange.com/questions/63286/… - but a really useful tip is to click on the help link under Add Comment, as this will provide you with a full list. This is a great posts/wiki by the way. –  Aaron Newton Dec 5 '11 at 21:48
Would it be wise, to allow trusted websites such as JS fiddle, to paste their iframe? e.g: <iframe width="100%" height="300" src="http://jsfiddle.net/fD5u3/embedded/" allowfullscreen="allowfullscreen" frameborder="0"></iframe> –  Val Mar 24 '14 at 21:43
Is there a reason that <u> is not on this list? –  user259718 Apr 29 '14 at 8:32
Can we get <center> on this list? –  Mew Oct 8 '14 at 9:28
@Mew No........ –  bjb568 Dec 20 '14 at 3:48
yes, alignment tags –  Zero Dec 30 '14 at 6:11
Is there a reason that <u> is not on this list?² –  Aurium Jan 10 at 17:26
@Mew: the <center> cannot hold :-) –  Roy Tinker Mar 16 at 21:30
Can I suggest adding the abbr tag (formerly known as acronym)? –  Rena Mar 30 at 23:38
I think the target="_blank" attribute needs to be supported on <a> tags. It would be really useful. –  TheMinecraftMan757 Jun 5 at 23:12

1 Answer 1

Allowed HTML Tags

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

<a>              - hyperlink.
<b>              - bold, use as last resort <h1>-<h3>, <em>, and <strong> are 
<blockquote>     - specifies a section that is quoted from another source.
<code>           - defines a piece of computer code.
<del>            - delete, used to indicate modifications.
<dd>             - describes the item in a <dl> description list.
<dl>             - description list.
<dt>             - title of an item in a <dl> description list.
<em>             - emphasized.
<h1>, <h2>, <h3> - headings.
<i>              - italic.
<img>            - specifies an image tag.
<kbd>            - represents user input (usually keyboard input).
<li>             - list item in an ordered list <ol> or an unordered list <ul>.
<ol>             - ordered list.
<p>              - paragraph.
<pre>            - pre-element displayed in a fixed width font and and 
                   unchanged line breaks.
<s>              - strikethrough.
<sup>            - superscript text appears 1/2 character above the baseline 
                   used for footnotes and other formatting.
<sub>            - subscript appears 1/2 character below the baseline.
<strong>         - defines important text.
<strike>         - strikethrough is deprecated, use <del> instead.
<ul>             - unordered list.
<br>             - line break.
<hr>             - defines a thematic change in the content, usually via a 
                   horizontal line.

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

Allowed Attributes

img Attributes

The following attributes are allowed on the <img> tag, but 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.

The attribute order is important! Using a different order (e.g., height before width) will strip the tag!

width="" (up to 999; do not include the 'px' extension)
height="" (up to 999; do not include the 'px' extension; see note above)

a Attributes

The following attributes are allowed on the <a> tag:


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.

  • You must enter the tags exactly as shown. Any deviation from this list—adding extra spaces, using single quote or no quotes, etc.—means the tag will be stripped.

  • We do not (and will not) allow <table> tags. Sorry. This is intentional and by design. If you need a quick and dirty "table", use <pre> and ASCII layout.
locked by random Mar 22 '14 at 12:36

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