Your initial page could use a half-page introduction that explains what the site is for - "This website/page is a jump-off point for a variety of short encounter descriptions for use with your favorite roleplaying game. Each of the short descriptions below links to a full page further detailing the encounter, including ideas to alter the content to fit your playstyle and the party's level. It also contains information to add to existing D&D content or official adventures.
The white background, while making the text clear and easy to read is a bit bland. I'd suggest left/right page runners - some sort of geometric design perhaps, some sort of quasi-medieval text font for headers (I prefer Morpheus, but also Diploma, Stonehenge, Pegasus or a similar fantasy font would be good). Perhaps a light parchment/paper background as well.
Of what I have read of the encounters, I think they're great ideas, and the modifications are thoughtful.
Death and Darkness - It's common parlance to call anti-paladins "blackguards" these days.
A Race Against Time - "The party is called upon to rescue a group of miners who were trapped underground after a cave-in." should probably be listed under the alterations - in fact, it might be good to have a section for each encounter to have a "Suggested Hooks" section prefacing the encounter. The primary motivation for this encounter is that the party is underground, and currently doesn't have a way back/out. How they got there isn't really important for the encounter. You should also probably expand the section on the secondary threats - digging out (which may trigger another cave-in), suffocation issues (especially breathing in the mold spores), how to possibly keep warm (without adding to the suffocation problem) and possibly food and water issues if the time to dig out/escape will be lengthy enough to matter.
Portal to the Mirror Plane - Don't force player action ("The party quickly realizes that they are in a dire situation.") - allow them to come to this conclusion themselves from the gravity of the situation. A better phrasing might have been "The situation is dire, and the defenders plead for the party's assistance."
The Sea Child Vision - The idea behind this is a good one, but I feel the way its handled is rather weak. There's no immediacy to the vision and it's too vague. I'm unsure how to "fix" it, but it needs more interactivity in some fashion. Perhaps after the initial intonation, the child might quest the PCs with a task/quest to "prove" they are the heroes of the vision and worthy of learning more about the impending disaster. While some of this is hinted at in the "possible modifications", there should be something in the encounter that makes it more than a simple pronouncement. This may be one where a hook would be really strong. Getting to the prediction might be the challenge in itself.
The Blood of the Planar - "The Planar from which the blood is harvested must be killed and its blood must be consumed within a day of its death" - I think you mean "must be manufactured within a day of its death", otherwise this makes little sense being found in an abandoned winery.
That's all for now, I'll try to get to the others in the near future.
Date YourName" text on the pics. (In some places, simply placing the copyright text on a pic is enough to establish copyright.) If you ever want a website to take down one of your images that someone posted without your permission, you're going to need to be able to produce the original image with the copyright text on it to even begin to prove that it's yours.
