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

//Specify spectrum of different font sizes:
var szs = new Array( 'xx-small','x-small','small','medium' );
var startSz = 1;

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

sz += inc;
if ( sz < 0 ) sz = 0;
          if ( sz > 6 ) sz = 6;
    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 ];
                                        }
}