I'm a designer myself, but I'm confused at what the question is asking. A designer can be many things. You can hire an interaction designer. You can hire an in

I'm a designer myself, but I'm confused at what the question is asking. A designer can be many things. You can hire an interaction designer. You can hire an interface designer. You can hire a website designer. I am going to assume you are speaking about an interface/web designer. Yes is the answer. When it comes down to it, your end users are probably going to avoid your service if they don't think it looks good. The amount of traffic you'd gain with a good design easily justifies the price of hiring a good designer. Now, the other thing is the word 'professional'. Professionals are very expensive. You're talking at least 10 thousand dollars for a decent project. If you go with someone who's a student or the like (cough), you'll get a better rate, but you won't get the timeline that a startup might need. There's the other issue. If it's not going to repay itself in traffic easily, then it's not worth it. In most cases, will it? Of course. The other option is DIY. If you take some time and learn your shit, you can usually do something that doesn't look half-assed. Now, moving on to the HTML bit. I do both. Many designers do both. Plain and simple: if you don't know markup, you shouldn't make mockups.