A WPF or Silverlight RichTextBox equivalent in HTML5 that would allow implementation of custom autocomplete feature on a TextBox

I am looking for an HTML5 equivalent of a RichTextBox control of the kind we have in WPF or in Silverlight. I have a Silverlight application in which I use a RichTextBox to create some custom functionality which shows, in a popup, a word starting with the letters matching the first few characters you type in the TextBox. Once this popup shows, you can just press the space key to get the full word typed in the TextBox without have to type the full word. When I built this functionality in my Silverlight application I used KeyUp and KeyDown events of the RichTextBox to respond to they keys pressed by the user. I also use TextPointer Class to get the letters being typed by the user when he starts typing a new word. I would now like to create a copy of my Silverlgiht application using HTML5 and some other library like React.Js. I would appreciate some guidance on which HTML5 control or elements I can use to implement this kind of functionality. Many thanks for your help. Please see the image which explains the functionality I wish to create in my web application.
As the image below shows as soon as the user types the a few letters (in this bec) a promt appears just below the letters typed which shows a word (because) whose first three characters match the letters typed by the user.

.An image to display the functionality I need to implement in an HTML 5 element or control
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David Johnson, CD, MVPOwnerCommented:
Textbox autocomplete is relatively easy

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FaheemAhmadGulAuthor Commented:
Thank you very much for your guidance. The link you provided was very helpful. However, I need to implement this autocomplete functionality in a TextArea or some other bigger RichTextBox like text control where you can enter multi line text. I would appreciate some guidance on how this would be accomplished in such a control.
FaheemAhmadGulAuthor Commented:
Many thanks. Greatly appreciated.
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