I tried a lot to put the element at the center, vertically and horizontally, to a div. Finally I used flexbox with providing value as center to justify-content and align-items.
Dynamically creating the html elements, specially input tag, was challenging for me when I was creating form for image-to-pdf operation.
While I was adding preloader, which show a loading page when site is loading, I fall in problem how to implement the loader in such a way that it will show only when page is loading. Then after a lot of research come to a solution windows.onload.
When I was adding image to title bar of the page, I fall in problem that it was not accepting img tag, then I was using that. Then after trying many other ways, I used link tag which worked.