WordPress users think that adding their custom HTML page or CSS style is more difficult, but if you find the exact Plugins then there is nothing hard to add you custom HTML pages to your WordPress Website.
Here I will show you step-by-Step process how to add you own Custom HTML page to you WordPress Website, Don’t worry you no need to have any expert HTML or CSS skills to do this Integration.
First login to your WordPress Website go to Plugins-Add New and install Just the Page plugin and activate it. This plugin disable the Header and Footer content for the page you select Display only page content.
I strictly recommend this second plugin Disable Visual Editor WYSIWYG, the importance of this plugin is it will disable the Visual editor in the page. The reason behind disabling the visual editor is, when you try to add your HTML content along with CSS and Scripting in WordPress page and when want to see you content in Visual Editor you may lose you CSS or Scripting code. So I recommend you to just edit the page with Text editor in page.
Go to Dashboard-Pages-Add New, the first thing you need to do is highlight checkbox to disable the visual editor for current post, highlight the checkbox to Display only page content and immediately click update changes to page. Now add your HTML content along with CSS and Scripting in the text field like shown in the figure, to check the preview click on the right corner top for Preview changes.
Note: if you have any external CSS file of your custom HTML page you need to upload the CSS file to Media-Add New and copy the uploaded URL link of the CSS file and add it to your HTML style text/css link.