How to know who is following me on Tumblr

It may be tedious, but it looks like you'll have to include the HasTags block in each of the section types.

Starting with the first you'll end up with this:

    {block:Text}
       <li class="post text">
           <!-- HTML layout for Text posts -->

        {block:HasTags}
            {block:Tags}
                <a href="{TagURL}">{Tag}</a>
            {/block:Tags}
        {/block:HasTags}

       </li>
    {/block:Text}

Where you currently have it appear just before {/block:Posts}, change it to be just before the closing </li> which appears to be what your post container styles are dressed in.

Don't forget to remove the block that appears just before the closing {/block:Posts} or you'll get the tags showing up twice.

It seems the real problem lies in the fact that embedded scripts are ignored in posts, most likely for security issues.

But it is possible to load JS within the description field when you go to "Customize your blog" (Kind of defeats the previous statement)

So you may be able to use a JS library (I think Write Capture might help) that allows you to write Javascript to specific parts of the document and use a parser to search and replace for phrases for example

[gist 1256020]
<div class="gistr">1256020</div>

Then you use a parser to deal with it. Or wrap Gist IDs in div element with say a class, iterate through all elements with that class and replace the id with the Gist using the Write Capture function So as you can see... it ends up going into a programming problem which is outside the scope of this site.