A List of All of My Problems in My Website

[Shivraj957]

I have many problems in my website.

1. Removing The .html Extension From The URL
   I need to remove the .html extension in the URL. I have one file named enter-your-username-and-select-your-region.html which has 2 URLs: https://shivraj957.github.io/indianmapwarmultigame/enter-your-username-and-select-your-region and https://shivraj957.github.io/indianmapwarmultigame/enter-your-username-and-select-your-region.html How do I remove the one
   with the ,html extension?
2. Redirecting One Page to Another
   My Github Pages site is https://shivraj957.github.io/indianmapwarmultigame but it doesn't show there as I don't have an index.html file. I want that when users enter https://shivraj957.github.io/indianmapwarmultigame they will be redirected to https://shivraj957.github.io/indianmapwarmultigame/enter-your-username-and-select-your-region
3. The Input Field is Not Required
   I want to make the input field required, how do I do it? Here is the code:
   <input type="text" id="Username" name="Username" placeholder="Username" autocomplete="on" required>
4. Changing The Height of The Select Box
   I want to change the height of the select box. I have written the following CSS code:
   select { height: 20px; font-family: 'Roboto'; font-size: 1rem; }
   Updating the font size and font family of the text written in the select box is working, but the height of the select box is not.

Please, respond to my questions. Thanks in Advance!