Why width property doesn't work on my table?

Answer 1

This is due to the behavior table where you cannot decrease its width less than the needed width for its content.

To be able to change this you need to set table-layout to fixed as by default it's auto and as you can read here:

Automatic table layout algorithm (this is default):

The column width is set by the widest unbreakable content in the cells Can be slow, since it needs to read through all the content in the table, before determining the final layout

and

Fixed table layout algorithm:

The horizontal layout only depends on the table's width and the width of the columns, not the contents of the cells Allows a browser to lay out the table faster than the automatic table layout The browser can begin to display the table once the first row has been received

main {
  position: relative;
  width: 80%;
  float: right;
  height: auto;
}

div.container {
  width: 95%;
  height: auto;
  margin: 0 auto;
}

main div.container>table {
  width: 10px;
  overflow: hidden;
  background-color: red;
}  

<main>
  <div class="container">
    <table>
      <thead>
        <tr>
          <th>Keterangan event</th>
          <th>Waktu event</th>
          <th>Lokasi event</th>
          <th>Lokasi event</th>
          <th>Cover</th>
        </tr>
      </thead>
      <tbody>
        <tr>
          <td>something</td>
          <td>something</td>
          <td>something</td>
          <td>something</td>
          <td>something</td>
        </tr>
      </tbody>
    </table>
    after adding table-layout:fixed;
    <table style="table-layout:fixed;">
      <thead>
        <tr>
          <th>Keterangan event</th>
          <th>Waktu event</th>
          <th>Lokasi event</th>
          <th>Lokasi event</th>
          <th>Cover</th>
        </tr>
      </thead>
      <tbody>
        <tr>
          <td>something</td>
          <td>something</td>
          <td>something</td>
          <td>something</td>
          <td>something</td>
        </tr>
      </tbody>
    </table>

   
  </div>
</main>

Answer 2

Is this what you need?...you set Table->width : 10px..... i guess you meant td->width: 10px..

.main {
 
  width: 100%;
  
 
}
div.container {
  width: 100%;
  height: auto;
  margin: 0 auto;
}
div.container > table {
  width: 90%;
  overflow: hidden;
  background-color: red;
}

<div class="main">
  <div class="container">
    <table>
      <thead>
        <tr>
          <th>Nama event</th>
          <th>Kategori event</th>
          <th>Keterangan event</th>
          <th>Waktu event</th>
          <th>Lokasi event</th>
          <th>Cover</th>
        </tr>
      </thead>
      <tbody>
          <tr>
            <td>something</td>
            <td>something</td>
            <td>something</td>
            <td>something</td>
            <td>something</td>
            <td>something</td>
          </tr>
      </tbody>
    </table>
  </div>
</div>