In the HTML spec, the intention of a 'space' character is it should exist only when a space is between two text characters, or when a non-breaking space entity is typed, ie
However, certain email clients see any space in code as a space character. For instance, let's say you want to ensure a child object in a table cell butts up to the edges of the cell itself. In most modern web browsers, this behavior would be assumed, however Outlook 2007, 2010, and 2013 would see any spaces in the code as a space character, thus creating gaps in the design.
There are multiple ways to combat this, the most straight-forward is to butt the child object directly against the opening and closing td, as follows:
<td><img src=".../images/design.jpg" border="0" /></td>
However, this has been known to not work 100% of the time. The only way to be sure of even butting-up without any gaps is to add some styles to the encapsulating TD's
<td style="font-size: 0px; line-height: 0px; border-collapse: collapse;"><img src=".../images/design.jpg" border="0" /></td>
This will ensure the encapsulated child object will be butted right against the edges. This can be done as well with a contained/nested table, but be sure the child table's font-size and line-heights are re-set; Outlook does a poor job of table inheritance, but other clients do inherit properly.