More PNGs decode, and content-width tables via CSS keywords

Two fixes for images and tables on real sites. A wide class of PNG images now display instead of showing as a broken-image icon: the compact 1, 2 and 4-bit PNGs sites use for placeholders, spacers, simple icons and loading cards, plus high-precision 16-bit PNGs, all decode now, where before only the common 8-bit form did. Grey card placeholders on news and content sites, which used to appear blank or broken, now render. And a table told to size itself to its content with the CSS initial or unset keyword now shrinks to fit like the reference browser instead of stretching to full width, so documentation and reference tables that use this pattern lay out at their proper width. Both are general fixes; pages that do not use them are unchanged, and the overall platform score stays at 70.