Whether it’s working with a difficult customer, or on a website that seems to have a million little details to remember, just focus on what the job is. You already know how to write professional website content, and that’s the most important detail for the job.
Sure, business writers should, ideally, know how to write professionally. It’s probably recommended for long-term success. However, the most common errors in business writing that I’ve seen don’t have to do with the actual writing itself. In fact, they are more about how you approach your writing and selling it to clients.