What do your customers use?
Have a look at your website on someone else’s computer or Mac, Laptop or tablet. How different does it look? Even if you are an “experienced amateur: when it comes to building websites, you can still get a shock at how different your site can look. Screen size is just one part of that. The browser you are using and (more importantly) the way in which the browser processes the code running your website can have a significant impact on the experience your website visitors have.
It’s a web designer’s job to ensure that your website looks good and functions properly across many different browsers. They should also perform browser compatibility testing to identify potential issues, and establish necessary fixes for those issues, before a website goes live.