To align text within table cells just use the CSS {{{text-align:<left|center|right>; display:block;}}} commands. These commands can be combined with other CSS commands.  The block forces the display of the span to 100% width of the containing element.

__Important: Don't forget to leave a space between trailing {{{%%}}} and {{{|}}}.__

__This__

{{{
||%%(text-align:left;display:block;)Left Heading%% ||%%(text-align:center;display:block;)Center Heading%% ||%%(text-align:right;display:block;)Right Heading%%
|%%(text-align:left;display:block;)Left Cell 11%% |%%(text-align:center;display:block;)Center Cell 12%% |%%(text-align:right;display:block;)Right Cell 13%%
|%%(text-align:left;display:block;)Left Cell 21%% |%%(text-align:center;display:block;)Center Cell 22 (longer for illustration)%% |%%(text-align:right;display:block;)Right Cell 23%%
|%%(text-align:left;display:block;)left%% |%%(text-align:center;display:block;)center%% |%%(text-align:right;color:red;display:block;)combine alignment and color%% 
}}}

__produces this__

||%%(text-align:left;display:block;)Left Heading%% ||%%(text-align:center;display:block;)Center Heading%% ||%%(text-align:right;display:block;)Right Heading%%
|%%(text-align:left;display:block;)Left Cell 11%% |%%(text-align:center;display:block;)Center Cell 12%% |%%(text-align:right;display:block;)Right Cell 13%%
|%%(text-align:left;display:block;)Left Cell 21%% |%%(text-align:center;display:block;)Center Cell 22 (longer for illustration)%% |%%(text-align:right;display:block;)Right Cell 23%%
|%%(text-align:left;display:block;)left%% |%%(text-align:center;display:block;)center%% |%%(text-align:right;color:red;display:block;)combine alignment and color%%