Adding an html table to wordpress is problematic job in visual mode. My old solution was to go into text mode and enter html code. However there is a new plugin called WP-Table. It allows you to a table directly in the Visual mode or import from a spreadsheet. Cool isn’t it?
WP-table plugin to add table
According to author:
This plugin creates and manages tables for wordpress. So you can post i.e. sport results in a fixed table format. You don't need to enter tables anymore in the plain text mode and the WYSIWYG editor didn't damage the table layout. The table layout can be changed via a css file and you can import a csv file as table.
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