Html/Css/Javascript Query

[djhutt]

I learned HTML/CSS/Javascript using out-of-date resources. I have noticed that the structure of the coding has changed in recent years. Examples are:

document.addEventlistener ("click", changeColour, false)
localStorage.getItem("name")
img.center(display:block; margin-left; auto)

I understand that the action to the right of the period/full stop is to be done to the item to the left of the period/full stop. But can someone explain that in better terms, please? What did this structure replace, and why did it come about?
This isn't an easy thing to just Google!

Much appreciated.