One of the many reasons is the browsers use different default values. e.g. when defining a width, IE will add the margin and the padding as part of the width, while other browsers will not include the margin and the padding. So to make your web page look the same you need to set the padding and margin not used to 0. So if you only use padding-left:5px; you need to set the others explicitely or they will use the default values.
padding-right:0;
padding-top:0;
padding-bottom:0;
or on one line padding:0 5px 0 0;
variable positions:
padding: top right bottom left:
all values other than 0 need px(pixels) after them.

Don't forget these values have to be within
styles="padding:0 5px 0 0;" in line
or in a style sheet for class name
.classname{
padding-right:0;
padding-top:0;
padding-bottom:0;
}

or id

#idname{
padding-right:0;
padding-top:0;
padding-bottom:0;
}