Compiler security can be defined as the preservation of robust properties, i.e., properties that hold in all contexts including adversarial contexts. Although attractive in concept, this approach runs into two challenges in practice. First, robust properties trivialize if the language has undefined behavior, which is a common feature of most modern language definitions. Second, proving the preservation of robust properties requires a simulation of target-language contexts in source-language contexts – a so-called backtranslation – which is very difficult when done syntactically. To address these challenges, we propose a new, \emph{semantic} approach to defining and proving robust property preservation. Our approach builds on the recently proposed DimSum framework for semantic multi-language reasoning, and reduces robust property preservation to a combination of standard compiler correctness and a backtranslation between semantic domains, which is considerably easier than a syntactic backtranslation.