Can a structured (text) document be effectively encrypted

My situation:

I want to encrypt an HTML document. My question is, if the following is known, is that going to weaken encryption?

  • It is a text-based document
  • It starts with <DOCTYPE HTML> or a few variants of
  • This is followed by an <HTML> tag, followed by a <HEAD> tag, which contains a number of ‘

Would this knowledge help a hacker to decrypt without the decrypt key? Would such knowledge make my documents effectively un-encryptable?

My apologies if this is a very simple question. I am a neophyte.