What your ticket most likely meant was to parse the Excel file, and display it on the client-side using HTML. And by mentioning HTML5, it most likely meant that you are free to use modern APIs rather than restrict yourself to older APIs in support for older browsers.
You can parse Excel with the server-side language of your choice and convert the data structure into a suitable format for rendering on the client-side using HTML. I also suggest you look for frameworks that allow spread-sheet like functionality, like jqGrid.
Also, before diving into the project, I suggest you clear things up with your client what that directive actually meant. Clients aren't usually techy, and it's up to you to translate their requirements into project specifications.