Sorry to hear some users are having this issue. I have an iPad Pro 2nd gen running current iOS and when I click the share menu on the right I see the following:  (this only works when a document is open)

Upload This File

Download Latest

Send as Attachment

Send as Email

Open In ...

Preview in Safari**(this is missing on some people’s iPad???)

Copy All

Dear Alexander, I too would love to be able to have my CSS and HTML open at same time. Thank you.

Dear Alexander, you may be aware of this, but just as recently as today Apple is now allowing "open in" for apps that execute code.  The APP DraftCode has just implemented it in their update today......but as a coding app they're not nearly as good as Textastic.....I LOVE textastic and hope this new open door means the PHP functionality will be included......if you did include it, I believe this would make Textastic the coding app choice for developers.  Thank you for considering this.  And, as always, I can't thank you enough for the great app you've provided!  Many sincere thanks!  -Louis