What special markup options are there in Discussions and how to use them?

YouTube Videos

Paste a link to a YouTube video and it will replace the link with a box containing the video title and thumbnail. Clicking the box opens an embedded version of the video.

http://www.youtube.com/watch?v=idt3HoPz9io

User Content

Add a link to any screenshots, artwork, videos, Workshop items, collections, guides, or Greenlight items and they will be given a preview. Clicking will pop up a preview.

Screenshot: http://steamcommunity.com/sharedfiles/filedetails/?id=117886695
Artwork: http://steamcommunity.com/sharedfiles/filedetails/?id=163988748
Videos: http://steamcommunity.com/sharedfiles/filedetails/?id=178460483
Workshop items: http://steamcommunity.com/sharedfiles/filedetails/?id=159186571
Collections: http://steamcommunity.com/sharedfiles/filedetails/?id=95744629
Guides: http://steamcommunity.com/sharedfiles/filedetails/?id=159458915
Greenlight items: http://steamcommunity.com/sharedfiles/filedetails/?id=93777661

Inventory Items

Right click an item in your inventory and click Copy Link Location. Pasting that link in a discussions post (or even a chat window).

http://steamcommunity.com/id/keavon/inventory/#440_2_1464041478

Trade Offer Buttons

A button to send a trade offer can be created by pasting a link to your public trade offers found under Item Inventory » Trade Offers » Who can send me Trade Offers?

http://steamcommunity.com/tradeoffer/new/?partner=88225129&token=0FwymFEe

Emoticons

I'm not sure if this counts, but you can type the code for an emoticon which will appear in your post provided that it exists in your inventory.

:tdealwithit:

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I'm sure there's more that I can't think of right now. I will update this if I find any more.

To do this simply go in you inventory, right click an item and click 'Copy link address' then post it in your topic/comment :)