I'm planning to apply H1 on our website for very pages but It's not possible because it will change the design as we all know H1 make the text big. So, I'm thinking to put h1 on the content but the text should not be change, it will look same font in the users eyes but it's h1 in Google. Is that possible? Is that legal or it's something sounds as black hat?
If you're worried about the size, just use CSS to make the size smaller. h1 { font-size: 20px; } Code (markup): I'm not really sure what you're trying to ask about it being "legal" though.
Probably black hat - if the text is just to emphasise keywords and not to denote an actual heading. BTW it's 2016 - don't think you'll come up with a trick that the nerds (said with admiration) and PhDs who work for Google and Bing haven't seen before and don't know how to detect. Save yourself a lot of angst and just play the game cleanly. If your site is any good you shouldn't need anything other than white hat strategies.