Command-+ style zooming in iOS Safari
Unfortunately this isn't possible with native iOS controls. A work around is a Javascript bookmarklet. First, bookmark any page and then bookmark another page. Then, choose bookmarks and edit them. For the first one, rename it the minus (-) symbol and change the URL to:

javascript:var p=document.getElementsByTagName('*');for(i=0;i&lt;p.length;i++){if(p[i].style.fontSize){var s=parseInt(p[i].style.fontSize.replace("px",""));}else{var s=12;}s-=2;p[i].style.fontSize=s+"px"}

Do the same for the other bookmark, but rename it the plus symbol (+) and change the URL to:

javascript:var p=document.getElementsByTagName('*');for(i=0;i&lt;p.length;i++){if(p[i].style.fontSize){var s=parseInt(p[i].style.fontSize.replace("px",""));}else{var s=12;}s+=2;p[i].style.fontSize=s+"px"}

Now to increase or decrease the size of text, simply tap the bookmarks.