my blog looks different on different devices

This may seem like a really silly question but in the world of blogging I am very new and very, very silly so please bear with me. My blog (www.peckpeckpeck.com/blog) looks good on my mac in firefox but is not readable on an iphone or in safari… I haven’t checked it on any other computers but have had no bad comments about how it looks from visitors so can only assume it’s good. I have just embedded a clip from youtube and on my computer it is beautifully centred but my husband sees the youtube box as left-aligned. I’m not sure how to fix this, can anyone help? Thank you in advance, Sarah