You can convert them all into placeholders by enabling Embedded content processing in Project Settings/File Types/XLIFF (1.1-1.2 v 2.0.0.0). However the default Tag Expression does not cover all tags. You can use the 'Preview file' option by browsing your file. If you still have some tags in plain text even after applying the said setting, add <.*?> in 'Tag definition rules' and try again. You have to remove your files, both source and target, and then add them again to apply the new settings. If you only remove target files and then prepare the existing source files, it won't work.
*Don't use the Close icon. Always press OK instead.

However, you should keep in mind that the xlf file structure may change after you export it from Studio. It always happens with TWS XLIFF Editor files for example. Also, some tags themselves may include translatable text. For example tags containing alt= (e.g. <img alt=...Click here.../>). If you convert all your plain text tags into placeholders, you won't be able to translate those translatable tag elements. Filter by text "alt=" and see if there are such tags. If yes, translate them first, then apply the above settings.

Stepan Konev said:

add <.*?>

For tag pairs you should use <name[^>]*> and </name> where "name" is the name of a tag. For example for <p> tags it would be <p[^>]*> and </p>.

For <div-class...> add <div[^>]*> with </div> and so on.

Stepan Konev said:

add <.*?>

For tag pairs you should use <name[^>]*> and </name> where "name" is the name of a tag. For example for <p> tags it would be <p[^>]*> and </p>.

For <div-class...> add <div[^>]*> with </div> and so on.