You are not doing Scrum
Based on your question, it doesn't sound like you are doing Scrum at all. In Scrum, the goal of a development team is to produce a potentially shippable product increment that can provide real value to end users. If you're splitting up sprints into design, HTML/CSS, database, and coding, then you are not in fact doing Scrum but are actually doing waterfall.
Instead, your product backlog should contain user stories that represent problems users are having that can then be taken by the development team and turned into working software. To do this, a single user story could be decomposed into all of the above tasks, database, design, HTML/CSS, coding, etc, but only for a single user story, or for a number of user stories that the team can commit to completing in a single sprint.
To clarify, in Scrum, all of these activities are done every sprint, not split up into different sprints.
As for your burndown chart, the chart itself should represent the amount of story points that are not completed yet in that sprint. This is based on the effort estimation the team does during the Backlog Refinement Meeting with the product owner.
A user story is not considered "done" until all of the tasks are completed that meets your acceptance criteria and definition of done. Again, this means design, coding, HTML/CSS, database, etc must all be completed in order for a user story to be marked as done.
Learn more about Scrum
Moments ago, eight of us in our office just completed the Certified ScrumMaster training from the Scrum Alliance. Before starting Scrum, I recommend a lot of focus on learning what Scrum means. A good place to start is Jeff Sutherland's book Scrum: The Art of Doing Twice The Work in Half the Time, as well as the Scrum Guide. If you're really serious, I highly recommend taking the Certified ScrumMaster course.