animating fonts styles with source text method but it was not smooth.

AE doesn't natively support variable fonts or stuff similar to CSS transitions as used by that web site, so you can't do it easily. You have to convert the text to outlines and use "morphing", i.e. copy & pasting different shapes into different shape path or mask keyframes.

Mylenium

You can't. Depending on what you actually wnat to do it would probably make a lot more sense to have separate lines and apply whatever effects and transitions. Otherwise you can of course always try to use text animatiors to disguise the transitions with blurs or text animating in and out.

Mylenium