Well, Axure is not a web page editor. If you want to take existing HTML and just make a few changes with full visuals, you are better off with a tool like Dreamweaver or even a good text editor like Notepad++.
Axure is intended to be a wireframe prototype tool which lets you quickly (or slowly) design, demo and test the basics. You can include full visuals though, including PNG, GIF, JPG, SVG, etc. You can also inject or refer to external CSS and Javascript if needed to help with styling.
For recreating the layout, look and feel of an existing web page or app, you can take a screenshot (or collection of screenshots) of a page and drag that image into Axure as a guide to then place, size, and style Axure’s widgets on top of it. I recommend converting the screenshot image(s) to a dynamic panel (right-click the image for popup menu, or CTRL+SHIFT+ALT+D on Win, CMD+SHIFT+OPT+D on Mac.) Then you can set the opacity to 50% or whatever, send it to back, hide/show the panel, move it front/back, turn the view on/off in the Outline panel to use it as a “onion skin” back and forth while you set up your page. Delete it or just hide it when done.
You don’t have to recreate everything on the page, either. You can oftentimes keep some things exactly as they are in the static screenshot and just overlay hotspot widgets for any different interactions you need.
You can also copy and paste images directly from a web page (if you have legal rights of course
) You can’t directly drag & drop–at least not in Windows but you can right-click-copy-image then right-click-paste in Axure. You can also slice or crop copies of your screenshot image (right-click any image) to use areas with identical visuals.
For HTML/CSS styled elements, you can use the browser’s Inspect tool to see the coding, get colors, fonts, sizes, treatments, etc. which you can then use with Axure’s widgets and styles to recreate things. Or you can use the graphics tool of your choice to create/copy anything then pull that into Axure. If a button or container has a complex design I will often take a screenshot, pull it into Photoshop, inspect the colors, recreate text to match, or remove text, or pull out / recreate portions of it or whatever… and then I save out layers to PNGs to pull in to Axure.
Something to keep in mind is that Axure converts pretty much everything to PNGs in the prototype’s HTML, so if you can create anything better/quicker/easier in a graphics app, that’s usually a better way to achieve final visual look & feel. The advantage of Axure’s built-in widgets and styling is that it is easier to resize, change text, styles, etc. --which is more of a wireframing activity anyway.
Good luck!