If you are creating mockups in Designer, you can use the output tools to export all of your design elements as resources that you can then use in a web design app. See Affinity Designer Workbook, page 360 for a tutorial (if you don’t have the book, it is worth buying. You can get it from the Designer website.)
As for using these resources in a web design app, make sure you make note of all the colors used in your design, as they will be needed for the exported content to match the color of any new content such as text or background fills.
By the way - If you have any content that you need to export as PNG (such as items with transparency) make sure to run the images through a minimizer in order to achieve the smallest file sizes. One resource is TinyPNG. Do the same with JPGs. While a Photoshop plug-in, you can go the the TinyPNG / Tiny JPG website and process up to 20 images at a time for free.