The <title> </title> tag is missing. It should enclose the text "Hello World".
The <h1> tag has not been closed with the </h1> tag. This means that the whole document will be in the large font heading style.
To test this you could copy this text from the current browser window and paste it into a new text editor document (e.g. Notepad or TextEdit). Save it as an HTML document then view it in your browser - you will then see the errors. See if you can edit the document to fix the errors.