Updated HTML5 page structure

Have been using non-standard <container> definition so far, but realised
I could substitute standard HTML5 <main> instead, so have done this.
Pages now validate in the W3 HTML5 validator - result!

In passing... noticed that <main> wasn't in the definition for legacy
browsers - and IE10 broke as a result!!!  So actually, IE10 compares
with *legacy* versions of Firefox/Chrome/Safari etc., not up to date
ones...  My despair over IE grows ever greater...