Some thoughts on requirements:

Sort should be on the text content of the <td> elements and not on (HTML) elements. Sounds obvious, but I saw simple tools that, when sorting, did sorted <a href="...">XYZ</a> before <b>ABC</b>...
Probably also obvious, but sorting means only on rows with within a <tbody> section, not on <thead> section. If there are several <thead> elements then... I would think that the corresponding <tbody> elements are sorted independently of the others, ie, every <thead> - <tbody> pair is a separate "sorting context". But opinions may differ on this.
I am not sure what to do with a simple table without the <thead>-<tbody> pairs. I would not be shocked if those tables could not be sorted :-)
The user should have a possibility to choose between ascending and descending order (usually through either a choice in the header using some icons, or by repeatedly clicking on the header).
Whether a table is sortable or not could be controlled, e.g., by special classes.
Bonus feature: possibility to automatically cut a large table to smaller chunks (say, 10 entries) and move from one chunk to the other via some extra user interface. (This is a bonus feature, ie, it may not be a requirement in the first round.)

Make table rows sortable #3483

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