The tags, <html> and </html>, are examples of paired tags. The second one is like the first one except that it begins with a / symbol. A pair of tags affects how a browser will display the text between the first tag and the second tag. Leaving out the second tag usually changes the display of the rest of the document.
A Redline. Hot Wheels car is one that was manufactured within the first ten years of production, 1968 to 1977. The term redline derives from the fact that, during that period, the cars were manufactured with a red stripe on the tire.
Now with that said, it's quite imposable to pinpoint the rarest Hot Wheels car. Yes, the pre-production Rear Loading Pink Beach Bomb is the most valuable, but there were about 200 of those made in various colors. From my professional opinion there are a few cars that are of the rarest.