Hi Hector,
Yes, I am aware that the span tag is causing the problem. I thought by adding a </span> tag to wordpress-popular-posts.php, the problem would be solved. But even after adding the </span> tag, it does not appear on my website when I view source.
I used the following codes in Use custom HTML Markup section:
1) Before / after Popular Posts:
[<ul class=”small-thumb”>] []
2) Before / after each post:
[
- <span class=”small-custom-frame alignleft”>
] []