Html – I am trying to make an aside menu with <ul>

I am making a little page as a project for school (don’t mind content, it’s in Polish). I have tried deleting CSS and it worked, but the look of it was just awful.

Here is the HTML:

<header>
    <img src="zdjecia/diablologo.png" alt="Diablo Logo">
</header>
<aside>
    <ul>
        <li><a href="bocz1.html">Pow</a>
        <li><a href="bocz2.html">Powrót do strony głównej</a>
        <li><a href="bocz3.html">Powrótony głównej</a>
    </ul>
</aside>

Here is the CSS:

body {
    font-family: Arial, sans-serif;
    margin: 0;
    padding: 0;
}

header {
    background-color: #333;
    color: #fff;
    padding: 20px;
    text-align: center;
}

header img {
    width: 350px; /* Zmienić odpowiednio */
    height: auto;
}

content {
    margin-left: 500px;
    position:relative;
    height: 100%;
    font-family: Arial, Helvetica, sans-serif;
}

aside {
    min-height: 110vh;
    background-color: #666;
    color: #fff;
    width: 2;
    float: left;
    padding: 30px;
}

aside ul {
    list-style-type: none;
    padding:0 ;
}

aside ul li {
    text-align: center;
    margin-bottom: 30px;
} 

content {
    flex-grow: 1;
    padding: 20px;
    margin-left: 230px;
    flex-direction: column;
}

footer {
    background-color: #333;
    color: #fff;
    padding: 10px;
    text-align: center;
    clear: both;
}

a {
    text-decoration: none;
    color: white;
}

p {
    text-align: justify;
}

h3 {
    font-size: x-large;
}

I tried making <a> as a button but still nothing. I just want to open second page linked to a specific <a href>. The link to the page is valid; it worked without CSS.