I don't think "professional" is the thing you wanna focus on, instead you want to make sure your portfolio is “communicative”. Building a portfolio is a design job like any other: you have to think of your audience and what information they need to see. I think a lot of people get tripped up 'cause they're like "I don't know what a recruiter is looking for". But the way that recruiters choose who to interview is the same as when you've made choices between multiple options before.

Here's an example: let's say you wanted to hire a baker to bake you a chocolate cake for your birthday. You would want to see examples of that kind of cake on their website or in their store before you hire them. So let's say you have three bakers to choose between: Baker number one is doing a lot of red velvet right now, baker number two has examples of vanilla and chocolate cakes, and baker number three is just doing exclusively cupcakes. Based on that, who would you hire?

All three bakers probably have the skills to make the cake you want, a chocolate cake is not hard to do. But only baker number two has the evidence that they can do what you need. You can't know what the other bakers are capable of without seeing it, and why bother taking a risk on them anyway when you already know baker number two can do exactly what you want.

Like the bakers, you'll be hired based on the work people can see. People will not fill in the gaps or assume you can do something you haven’t shown - you have to be explicit. You also need to talk about your role on a given project, your responsibilities, your creative process and rationale, and the outcomes of your work if possible. This is especially important for juniors too with little to no experience, talking about your role within your team (either at work or school) and how to solve problems independently will help visitors to your portfolio understand what you’ve done and what you’re capable of.

Otherwise, you have to implement good web design practices, especially as a designer (UX, UI, and accessibility). Writing in a professional (which can also be warm) tone, no typos or broken links are also important. In the best case scenario, the portfolio itself becomes a portfolio piece, and a representation of your design skills. So focus on the experience you want your audience to have and what you want them to learn when they visit your site, and how you can best do that for them.