I did already find a post about using the <hr> tag to insert a line break, but when I looked up the tag on the w3 website (http://www.w3schools.com/tags/tag_hr.asp) it says that all attributes of the tag are not supported in HTML5. Obviously I want to make my website HTML5 compatible, so what would be the best way to insert a visible horizontal line?
Thanks
You can still use <hr> as a horizontal line, and you probably should. In HTML5 it defines a thematic break in content, without making any promises about how it is displayed. The attributes that aren't supported in the HTML5 spec are all related to the tag's appearance. The appearance should be set in CSS, not in the HTML itself.
So use the <hr> tag without attributes, then style it in CSS to appear the way you want.