PHP programmers can be tempted to consider only the server side of the website equation and leave the client side to the designer. Your job, as a PHP developer, is to make sure that the requested actions happen efficiently, and this does not appear to be a client-side problem.
When a customer expresses that the site doesn't look very modern or snappy, as a server-side developer, it is tempting to think that this is a problem for the designers, as they have traditionally had most of the control about what is sent to the client, and you have traditionally only been asked to supply an HTML skeleton for them to wrap the design around.
However, more and more code these days is shifting onto the client side of the transaction. And as a result, developers are taking a stronger interest in how they can affect data on the client side, and how they can help the designer to make an impact.
There are reasons why you, as a developer, should be interested in what happens on the client...