Always Be Learning
Most business owners will have no idea about how websites are built and hosted, and trust their developer to make the right decisions for them.
But the web design and hosting industry moves fast, and if developers don't keep up with the latest developments they will fall behind best practices.
A solid understanding of the fundamentals of HTML, CSS & JS is the best medicine for this. While platforms and frameworks and libraries come and go, websites ultimately are a combination of these technologies. If you understand how they work, you will always be able to make good websites.
SEO techniques are always changing as our search engine overlords adjust their algorithms. There are lots of tried and tested techniques, but the basics of a well written website with quality content is the platform that is always required for long term, organic SEO growth.