I just posted in one of the sections (diseases, etc...) and included two links to a tool I used. In the preview the links didn't show up and instead there was a large empty space, much larger than the four lines the links used up. I tried a few times before I gave up and posted the message anyway. In the posted message the links were also not there, and I had reworked the message so the large gap was at the bottom of the message now so it wasn't as noticeable.

Is there a forum rule prohibiting the posting of links? Or is there a special way to do it so that they show up in the message?

Here is one of the links:
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See, it didn't show up...

@jbclem It shows up just fine for me. It's at Amazon, Carson MicroMini 20x pocket microscope for $10.99. Since I can see it just fine, I think it is your browser that has an issue. Clear all of your cookies and history, make sure it's updated and restart it.