Question

Here’s my jquery code. If you can’t see any flaw in it, please proceed to see JSfiddle here at http://jsfiddle.net/J4ZgB/3/ Thanks.

$(function(){
    $("#a").click(function(){
        var s=$('#p_content').css("font-size");
        s+=4;
        $('#p_content').css("font-size",s+"px");
    })
})

Answer 1

You had two errors.. fixed code:

$(function(){
    $("#a").click(function(){
        var s = parseInt($('#p_content').css("font-size"), 10);
        s += 4;
        $('#p_content').css("font-size",s+"px");
    })
})

1. You need to parse the numeric value of the font size, which contain “px” – luckily, parseInt takes the first digits in a string and converts them to number ignoring the rest of the string.
2. You tried to set font of element with ID p_cont (in jsFiddle) while the real ID is p_content.

Updated jsFiddle.