Display user input on different form

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Javascript

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I'm creating a website and would like to display the current user like this:

Welcome (USERNAME)

The login form is login.html and the main form is main.html but I would like to get the entered username from the previous login form and display it on the main form. Please could anybody help.

What I have tried:

login.html:

<form class="box" action="main.html" method="post">
  <h1>Login</h1>
  <input type="text" name="" id="username" placeholder="Username">
  <input type="submit" name="" value="Login">
</form>

main.html:

<b><span id="username"></span></b>