Beginner question here; I’m struggling with positioning containers. I’m not really sure what concoction of declarations to use in order to format my container sizes and locations without using flexbox, grid, or tables. From what I’ve read flexbox and grid aren’t supported on ancient browsers so I may need to avoid those for certain web projects depending on the intended user. I’ve also read that tables are poor form and should be avoided.
Shopware 5 doesn’t raelly have a grid system like bootstrap. So you have to build your own little grid ( if you need it ). Shopware 5 uses https://arnaudleray.github.io/pocketgrid/ so you can take a look how to build your own tiny grid.
For flexbox and IE 11 you may use Autoprfixer ( https://autoprefixer.github.io/ ) which adds the prefixes for browser compability. However, flexbox is supported by all modern browsers. You can check the compability at https://caniuse.com/#search=flex
With css grid you have a few more problems with IE 11, just google for „css grid IE11“ - But you can still use it, with a few fixes/polyfills/what ever
Buildung a grid/website with tables is one of the badest things you can do in todays development world 
It was pretty common years ago - I also started with table layouts 15 years ago or so. But today it’s an absolute No-Go to build a grid with tables.
Tables should be used for what it is: A table which shows content in a table and not a grid.
You may also check out the docs about shopware => https://developers.shopware.com/designers-guide/
And make sure to take a look at the best practise theme development article => https://developers.shopware.com/designers-guide/best-practice-theme-development/