Like most word processing programs, Wattpad has the delightful ability to let the writer change the character of their words via HTML alternations.
That's fancy talk for you can bold shit.
Or, rather, shit.
That being said folks, don't go crazy and do something like this (and yes, I have seen this happen):
OMG DID YOU SEE WHAT THAT BITCH WAS WEARING?!!?! WHAT A HO.
Thank you for reminding us just how juvenile you are.
The only people who go that crazy with the text alternations are prepubescent idiots (ah, the memories of that ancient age) or cats. Mainly because both of those attack keyboards with surprising ferocity.
At least, that's what I hear is true about cats. My dog sort of just stares at me; he don't need no man/keyboard/thing. That didn't work. *Sigh*
Anyway, writers shouldn't use cheap tricks to make their writing "better" and altering the text is a big cheap trick. There are, however, times when you should alter your text.
CAPITALS: When you don't have access to bold (really) or if it's a company title or something that is supposed to be in capitals and only in capitals. Like APPLE if Apple didn't want to be all "we're cooler than the Man, or at least we think we are. Artists unite, dude.". That means that it's inappropriate to have a character/narrator yell in capitals. DON'T DO IT! See how annoying it is? It's just jarring to the eye. *Shudder* I want to read your text, not feel violated by it.
Bold: When something needs to be emphasized that cannot be done in text. That means that if it's already emphasized in some way, leave it the hell alone. For instance, scene where someone yells, "Dad!" is redundant. The exclamation is a bold sign. You should never bold anything that is already highlighted by an exclamation point, unless it's a factorial because that stuff is awesome. You can bold something if it's part of a sign or if it's something really important. But it has to be used sparingly -- bold text is a bit of drug. If you use too much you're left dizzy, your eyes jump all over the place, and you become addicted. It's better to take a tiny hit only once in a while so you feel its full effects (Wow. I sound like a druggie). Can also be used to refer to texts when italics are unavailable.
Italics: When you are citing stuff, telling a character's thought, or when referencing certain works. Italics are a bit less powerful than bolding so it's alright to use them more often. That's why you should use them when referring to words from a different language in text (unless you're lazy like me and just let it slide. M'en fiche). Thing is that if you highlight every word that the person emphasizes it looks like hymn song lyrics. Ever seen the lyrics for the Psalms? It's like "Something something something, something something else. Ref." (The italicized bits are meant to show the singer where to emphasize the text, by the way.) You don't want your story to look like that.
Combination of two or more of the above: Never. Ever. Ever. Ever. I'm scowling at you preteens.
Bottom line, you can alter text. Just take it easy. We're not all hardcore text-altering druggies like you (Ooo. Like magic mushrooms only for text... So magic underlinerooms? No, that doesn't work either. Dang it! I'm so not on form today).