Create flexible web page layouts for all devices
The myriad screen sizes today require responsive web designs. Adaptive and responsive designs accommodate content more gracefully in general, but they won't save you from a lack of testing with actual content.
In order to be successful, your layouts, navigation, and text containers must be flexible both during the design process and long after deployment.
Use progressive enhancement principles to design for accessibility and cross-platform compatibility.
Design with existing content.
If you're redesigning and you have content (even obsolete content), you could use that to begin with. In any case, if you’re designing a system of substantial size, you’ll need to fit the design to content as well as the content to design, because the website design and content must grow together.
If you have no content yet, borrow some similar media from other sources and assemble it inside wireframes, prototypes, and other mockups. You can also print out, cut it up, and rearrange content items on a table to imagine how your page layouts should handle it.
This exercise can also help you better understand the types of content you'll need to address in order to make the website competitive and comparable in your industry.
Use placeholders only if you have to, and then only in the first stages of low-fi wireframe design communication.