//Specify affected tags. Add or remove from list:
var tgs = new Array( 'div','td','tr');

//Specify spectrum of different font sizes:
var szs = new Array( '12px','14px','15px','16px'); //var szs = new Array( '11px','12px','small','medium','large' );
var startSz = 0;

function ts( trgt,inc ) {
 if (!document.getElementById) return
 var d = document,cEl = null,sz = startSz,i,j,cTags;

 //If the number passed to the function is 5 then reset the size of the text to default
 if ( inc == 3 ) 

   sz = 0;

 //else add or subtract from the number to determine the new font size
 else
 {
  sz += inc;
  if ( sz < 0 ) sz = 0;
  if ( sz > 4 ) sz = 4;
} 
 startSz = sz;

 if ( !( cEl = d.getElementById( trgt ) ) ) cEl = d.getElementsByTagName( trgt )[ 0 ];

 cEl.style.fontSize = szs[ sz ];

 for ( i = 0 ; i < tgs.length ; i++ ) {
  cTags = cEl.getElementsByTagName( tgs[ i ] );
  for ( j = 0 ; j < cTags.length ; j++ ) cTags[ j ].style.fontSize = szs[ sz ];
 }
}